So you'll know it's supposed to be funny.
If you've ever watched the the videos of the Big Bang Theory sitcom with the laugh tracks removed, you'll notice it feels a lot more like a group of people being shitty and verbally abusive towards each other and it's not all that funny without being told to laugh.
Why go through the trouble of writing an actual funny dialogue when you can just tell the viewers it's funny and they'll laugh?

They started using laugh tracks back in the 50s, but they became the standard in the 80s due to the attack on Fran Drescher. At that time, sitcoms were still commonly recorded in front of a live studio audience.

In 1985, she and her husband were brutally attacked in their home by two men who had stalked her from a live taping. In response, the studio went to a closed set for security and hired Central Casting “laughers,” that were eventually replaced by a laugh track. Other sitcoms followed suit when studios saw the ratings and cost benefits to a laugh track over taping in front of a live studio audience.

https://www.nzherald.co.nz/entertainment/how-fran-dreschers-stalker-ordeal-changed-sitcoms-forever/VUD7L6NOIKJIMB5S6GG3KWVXUA/

Because people watching TV alone are more likely to laugh, and are more likely to rate the show better.

https://en.m.wikipedia.org/wiki/Laugh_track

MASH had a laugh track that the creators hated, but the network insisted on. As a compromise, they agreed not to use the track during surgical scenes.

https://www.mentalfloss.com/article/56777/why-did-mash-have-laugh-track

Or is a by product of its former format, the live laughs with a crowd while filming?

This is the reason. Television comedy derives from stage shows where the audience sits in one direction from the stage.

A lot of early television comedy programming was often from variety shows, where the live studio audience is an important feedback mechanism for the actual performers. A standup comic needs a laughing audience to respond to (and often, so do other stage performers, including sketch comedy).

So television comedy comes from that tradition, and a live audience was always included for certain types of programs. Even today, we expect variety shows to have audiences. For example, John Oliver's show without an audience felt kinda weird while that was going on in 2020. And even some pre-filmed sketch comedy shows, like Chappelle's Show, would record audiences watching the pre-recorded sketches as part of the audio track for the broadcast itself, while Chappelle himself was filmed essentially MCing for that audience and those sketches.

So sitcoms came up on sets with live performances before studio audiences, just like sketch comedies and variety shows or daytime talk shows. That multi camera sitcom format became its own aesthetic, with three-walled sets that were always filmed from one direction, with a live audience laughing and reacting. Even when they started using closed sets for safety and control (see the Fran Drescher stuff linked elsewhere in this thread), they preserved the look and feel of those types of shows.

Single camera sitcoms are much more popular now, after the 2000's showed that they could be hilarious, but they are significantly more expensive and complicated to shoot, as blocking and choreography and set design require a lot more conscious choices when the cameras can be anywhere in the room, pointed in any direction. So multi camera still exists.

I believe its a byproduct of originally being filmed in front of a studio

It's needed for some shows like Big Bang Theory to know when a joke was " funny "

QI (the British panel show) discussed this in an episode during social distancing where they had to perform with no audience: https://youtu.be/EKVD3n6Atl0 (it's the first topic of conversation, not the whole episode of course)

My favourite bit is:

Alan: "I had a radio show in the late 90's, and we were so funny that the people at the BBC comedy said we could use those laughs on nearly every other program we make. [...] That was the best compliment I've ever had in my whole career. 'We've kept your laughs, and we're using them on other shows'."

I know that on FRIENDS, they filmed the show with a live audience. They used the audience to figure out if the jokes were funny enough and drew a lot of feedback from them.

The show definitely also used laugh tracks as well.

Some shows use an obnoxious amount of laugh tracks though.

This one guy invented "the laff box", a fairly complex machine with a wide variety of laughs that you could put together however you wanted to, and it was so successful in getting the audience to laugh that every show used it.

And that one guy would often be the only one who knew how to operate it, and could basically play a piano of laughs to suit the mood or whatever for any tv show.

It's a pretty great story if you're interested

https://www.youtube.com/watch?v=mvzPDZrpTHo&t=0

99% Invisible did an episode on this.

It's a great listen. Basically it makes audiences think the show is funnier, or rather it gives audiences permission to laugh out loud alone.

https://99percentinvisible.org/episode/the-laff-box/

https://en.m.wikipedia.org/wiki/Sweetening_(show_business) gives a quick rundown.

In television, sweetening refers to the use of a laugh track in addition to a live studio audience. The laugh track is used to "enhance" the laughter for television audiences, sometimes in cases where a joke or scene intended to be funny does not draw the expected response, and sometimes to avoid awkward sound edits when a scene is shortened or more than one take is used in editing.

I think it’s a combo of the two. I watch a lot of panel shows and the first thing that comes to mind is pandemic filming. It actually was a bit awkward at first not to have the studio laugh because it was traditionally filmed with a live audience. That’s still half an argument as certain shows will have cues for the audience.

At the end of the day we are very social animals and appreciate feeling emotion together so if we find something funny/scary/exciting we get positive feedback when others around us (at least sound like) they feel the same. It’s the main reason a decent amount of people enjoy going to the movies.

This will fall flat when laugh tracks are added over frequently; but when used properly (and with actually good jokes) they encourage the audience to fully engage.

Imagine the embarrassment of being the only person to clap/laugh/scream at a performance; thats the intended buffer for ‘canned’ responses.

Edit; when I started this comment there was only one response. What I cover was also brought up by other commenters, but I’m slow at typing while I’m sleepy (RIP)

An interesting case (from a book which I unfortunately can’t remember the name of) from when Jack Benny’s career transitioned from radio to tv: he hated the laugh track, so much so that he demanded it be cut way back and lowered in volume. He also utilized it in an unexpected way: when he had a live audience in certain cases, if a joke or gag got an unexpected big laugh that he didn’t think deserved the reaction, he’d fill in a laugh track with a more muted response.

It's not added to an existing piece (or play) but written with the kind, length and ibtntensity of laughter. There are videos with the laugh tracks removed where you can see the small breaks of the cast waiting for the laugh track to stop. Not just is there a timing difference with and without the laugh track, no, jokes themselves work differently. Meaning: without a laugh track you might not be able to tell joke from serious because the guide is missing allowing to laugh (e.g. about the misery, clumsiness, idiocy, etc of a character). Sitcoms without laugh tracks expect compassion to do its magic and is probably much more closely tied to cultural context of the work. Nobody has to tell you what's funny with Malcolm in the Middle or The Office, you know, because you once experienced (or observed) similar.

It's because they think you're too dumb to get the joke and wanna let you know that something funny happened.