It's a good way to do business for small companys as people who wouldn't usually buy the games think that they can pay a lower fee and be getting a good deal. So they buy the games for say $5, $5 that the devs wouldn't have got if all the games singularly were $5 each. Thus the devs sell way more games than they would if everyone who wanted them paid full retail for them. It wouldn't work for say the next Grand Theft Auto, as everyone wants it but the majority of people wouldn't pay much for it. This means the devs would lose out as a lot more people would pay retail whatever it cost, purely for the name.
That said, I'm really enjoying Aquaria and might have a bash at some of the others tomorrow seeing as I'm off.