How come some linux users don't have home directories?

Question

In linux I only see a /home/[[me]] directory. However, I know there are at least 20+ other users (root, www-data, etc..).

How come some users don't have user directories - and is it possible to create new users without a home directory?

Answer 1

Many of these users are created as system users. On Debian/Ubuntu, you can do that with adduser --system. If you only want to avoid creating a home dir (not creating a system user), you can use adduser --no-create-home.

I'm sure, there's an equivalent for non-Debian systems (with useradd etc). The useradd manpage tells me:

-M Do not create the user's home directory
-r, --system Create a system account.