Redmine

So I'm struggling with setting up the apache side properly with redmine. Most of the install guide explain apache to give www-data access to redmine and stuff... Which seems breaking the permissions on many level for redmine user to work and database stops working after I do the apache configuration steps but that's another subject.
However I started looking into to understand how and why I need apache to actually use redmine itself.

Because if apache is disabled and turned off ruby still make redmine available with IP:3000 port.

So how does this work? Can someone explain or guide me to some documentation please?