While most computers are personal computers, which have a single real human user, you still have to run a lot of untrusted programs, like the Internet browsers or whatever programs you might download from dubious sources.

While perhaps the term "user" is no longer the best, there is a need even more than before to run programs with limited rights, corresponding to the rights of some pseudo-users, which should not be able to access or modify anything belonging to the real human user, unless a special permission is granted.