It's a bit of the same idea as "init", but for users. When a user logs in, you somehow need to know what to do next. What does it even mean "to log in" if not

It's a bit of the same idea as "init", but for users. When a user logs in, you somehow need to know what to do next. What does it even mean "to log in" if not "run a program with my uid". Typically,you want to spawn a process that would allow the user to in turn run other processes. That's what the user default shell is. Trivia: changing the user default shell to /bin/false has been used quite a bit as a way to disallow a user from ssh into a machine (there are specific other steps to take into account when doing that which I won't cover here)