It's way more than this. Some may argue about how useful all of these features are, but Steam client is also a store (with lots of personalizations), a social network (forums, text and voice chat, personal profiles etc), a mod hosting platform, a game review site, a digital marketplace (cards and items. You may be surprised how popular this aspect is). It also does the controller remapping, manages cloud saves and screenshots/videos, allows to store games at several locations at ones. It provides games related news, checks for updates on all of the installed games.

Again, one may argue how useful all of this is, but it is there, and most users likely have the app on a OS startup anyway.