IMHO I think and have always thought the reason for Valve doing it in the first place is that they are scared about all the vendor specific stores which is poping up (of which they are one and one of the bigger ones but there could exist an even bigger more obvious choice) - As iTunes store, Google Play, whatever Microsoft calls their / Live.
_If_ Microsoft made a successful game delivery platform / sold all software signed through their own store Valve would have much less left.
If they rolled and had their own OS and people actually used it that would be less of a problem for them.
By now and since the Steam machine reveal GMG have started their own software with a similar purpose as Steams in that it keep track of your friends and let you upgrade your games and such but connected to their store which use all DRM free content. Since I want everything in one place that may not prevent me from using Steam but it's sure a more attractive option from a technical stand point. And as said the reason most of us prefer Steam over say Desura or something such is that it's biggest and (possibly?) have the most content (most popular if nothing else) - It's not something you can take for granted though.