The trading cards are part of the Steam economy. It's a money-making ploy. I don't know if it works or not, but it probably does.

If they dropped everything that isn't about selling games to players (Steam Matchmaking, social features, the Steam economy especially wrt CS:GO and Dota) I don't think Valve would be able to sell and host games as cheaply as it does, and I think more people would pirate games rather than getting them on Steam.