The benefit of a class action suit is not necessarily for the users to profit, but for the company to be punished. If I sue a factory for contaminating the drinking water and win $100,000, this is nothing more than a slap on the wrist. They will continue to do business as usual and consider my settlement the cost of doing business. But if I get my entire town to sue them for $350,000,000, then maybe they will wake up and change their attitude. This is the power of class action.
Valve could intentionally be violating their own terms and selling our credit card numbers to shady companies subscribing us to magazine subscriptions without our permission. If they have determined the risk/loss value to be less than the value of selling our info, guess which one they will choose? Class action makes sure the penalty is always severe. It's no different than if stealing a car got you 2 hours in prison, or robbing a bank a $500 fine. These punishments are too low for us, just as individual lawsuits are too low for them.