Sometimes i need to hand info to someone, or paper is just nearby, or i need to draw a diagram.
I do have an ipad, but if you are brainstorming with other people, they don't always know how to use it/touch the wrong thing.
All other notes are digital, because i am bad at keeping track of pieces of paper.
Everyone is part of the server they signed up with. Federation allows everyone to see each other's posts, filtered through your own server.
If another server defederates yours, their users essentially can't see your (new) comments/posts. But you can see most of theirs.
It is not necessarily a 2 way street. Continuing the example, your server can choose whether or not to defederate back. If it does defederate, you'll no longer see any new comments/posts from the other server.
A much better, longer post about this can be seen here.
And another one here