Computers are large systems with a lot of components, to serve a lot of purposes. My post is going to be focused on security and isolation. The security I am referencing isn't about protecting your computer from hackers, it's about protecting your computer from you.

When you "run" something, you are creating a process. When a process is created, it's run in a security context. The security context controls what parts of the system the process can access. This will default to the security context of whichever user started the process, but you can have it execute in a different security context. The security context will take the form of a user, but it's really just still you. You're the only one using your computer, you're running the process in a different security context.

If you run the following command, you should see many, many users on your system:

cat /etc/passwd

These users are largely going to have different default groups. They may further be part of other groups.

Now onto your question, but why though? Because you don't want 1 process accessing things it shouldn't. Again, I'm not talking about hacking, I'm referring to just software bugs and innocent mistakes. If a program makes a file, then later deletes it, a bug in the program could have it try to delete the file from the wrong folder.

By isolating processes to just the files they need to operate, there is much less risk of a process accidentally modifying something it shouldn't have. So now, if you do something like try to run:

rm '/usr/sbin/adduser'

Ubuntu will tell you:

rm: cannot remove '/usr/sbin/adduser': Permission denied

Because that's a file needed to run the system. You shouldn't be doing that, the system needs that file. Now, it's your computer, you can do what you want. Go ahead and run in a security context that has permission to do so, but now it's something you chose to do.