There are another ways to play online and for free, but people want to be a part of the fashion and re-buy games to look VIP. Bitcoins can buy more interesting things than reselled games on Steam.
Basically the main purpose of users purchasing games in steam is simple. Unlike those games that you were talking, yes you can consider some of them are good but when it comes to steam games comeone dude they are just too good and take note there are also free games from steam , but usually people bought steam credits not to download a game but to enhance their game experience.(eg. Dota2 Items)