Yes. One use case is application sandboxing; the users aren't separate people, but separate programs. Eg on Android, each app is a "user", and Linux filesystem

Yes. One use case is application sandboxing; the users aren't separate people, but separate programs. Eg on Android, each app is a "user", and Linux filesystem permissions are used to control what the apps can do. But there are also a few instances where you still see the different-users-ssh-into-the-same-server model; for example that server might be controlling a cluster, and have users ssh'ing in to do otherwise-slow computations.