Starting today, Steam members can make $5, $10, $25, $50, and $100 deposits into their Steam Wallet, to be used for purchasing (you guessed it!) games on Steam.
If you're wondering what possible reason you'd have to use Steam Wallet, given that it still makes more sense to purchase a $7.99 game for exact change rather than pumping $10 into your Steam Wallet using an identical payment method first, you're not alone. Valid questions like these have already been posted in the comments of the official "Steam Wallet Beta Now Available" announcement.
While Steam Wallet Beta's usefulness may not be readily apparent its current state, it could eventually serve as a gateway for prepaid Steam cards and open the service up a lot more for credit card-less and younger gamers.
I personally think one of the reasons is for refunds. Force people to rebuy on steam and not give cash refunds. (like instore credit)