Aside of regular games, Steam offers also a Steam Apps -- apps, programs and utilities that are deeply based on Steam's core -- like for example live wallpapers.
These are utilities that simply cannot exist, work or run without Steam. The problem is that when you uninstall Steam, the uninstaller (or Steam itself) doesn't give a fuck about Steam Apps. Neither uninstall them nor warn users about them.
Next time you restart your computer after Steam uninstallation you're punched into face with a lame messages like this:
And the best part of this is that uninstallers of this side-utilities... are also based on Steam. And since it is gone from your computer you actually have to... install Steam again only to clean mess it left in your system (only to uninstall these utilities).
Morons at Valve... I salute you!