Apple - why is user 'nobody' listed as a user on my iMAC?

There are a lot of places in the OS where something has to be assigned to some user account. Each file and directory must be owned by some user account, every process (program) running must be running as some user account, etc. The "nobody" account serves mostly as a placeholder for files, processes, etc that don't really belong to any "real" account (even one of the other system accounts in the list).

Essentially, the "nobody" account serves as a way to restrict permissions and access to/from things that don't really belong to any "real" account.

It's also not unique to macOS at all; you'll find it (or an equivalent) on any unix/linux system, and probably many other OSes I'm less familiar with.

Those accounts are for systems processes and to restrict access to things like files and resources by group or user.