The distribution sets them up so that the user doesn't have to know about them at all. E.g. the user doesn't have to know about CUPS in order to print. (Although I would be interested to know how many Linux users have succeeded in printing without knowing about CUPS.) Of course, that could also be made more intelligent - some services could be started when they are needed by default (I thought that was the point of upstart) and automatically moved to or from autostart on boot depending on how often the user needs them.