Which is really only used in the americas. Europe/Asia doesn’t use it, except in specific circumstances where the child wouldn’t be eligible for citizenship elsewhere. But even that is only due to treaties set up to prevent stateless people. If the child would have citizenship elsewhere (like in America), the European/asian country would tell them to apply there instead.

Basically: Resident enfranchisement. It's weird, when people born in our country and having lived here their whole life can't vote outside of local elections. My own father, for example, had a Dutch background, and was never allowed to vote in federal elections until his death. (Neither he nor I even spoke/speak a single phrase of Dutch)

Yes, things have gotten somewhat better and easier with applications for citizenship, but that there are hurdles like that to begin with, is a bit.... weird.

Both jus soli (citizenship by birth) and jus sanguinis (citizenship by blood) exist more for historical reasons than because one is better than the other. Both are simply a way to try and make citizenship a more clear-cut thing, because it's as close to being a made-up thing as you can get, especially in cases such as parents having a different nationality to the child (which is even more confusing when both parents are of different nationalities).

Jus soli is more common in the Americas due to various factors, including an incentive towards immigration from richer countries during colonial times and the various movements towards emancipation of the enslaved peoples a few centuries later, but the fact remains that neither system is any more arbitrary than the other. Jus soli is often favored because it simplifies things like immigration and asylum seeking and reduces statelessness, which is still a significant issue that affects millions of people worldwide, mostly around war-torn areas.

As mentioned in another response, enfranchisement is also a very important issue that jus soli resolves, although a significant part of it is also due to other, unrelated citizenship laws that may not necessarily conflict with jus sanguinis.