"User" in a modern Linux system is just a weird name for "security domain". Many programs run as their own user to limit their ability to attack the rest of the system if they get compromised; and limit the ability of a different compromised component from attacking them.

My desktop, on which I am the only person with an account, has 49 "users", of which 11 are actively running a process.

At work, every daemon we run has a dedicated user.

On android, every app runs as its own user.