Special said:
I am afraid this is false, these are just pre-cached images/thumbnails/banners/etc. nothing more, nothing less for the games in your library so that the images load quicker when you view something, I'd much rather downloaded them on demand, when needed, when viewed then have them all pre-download when I run Steam (because I might not ever view them). This is a problem for many who have large amounts of games tied to their account (think 3000+). Lots of sources about this issue too.
It worked for me, so it is true