Why? Because -- and get this -- games sell more when they're on the Mac. Games that have added a Mac version since Steam went live on the Mac are seeing a 15 to 20% sales increase. That's actually for both platforms -- most Steam games available for the Mac share ownership on both Mac and PC, so they're not really tracking sales of one platform or the other. Still, having a game available on the Mac is a big bonus, as not only does it make Mac players possible, but it drives attention for both platforms (as you may have even seen here on our site -- when we post that a game has finally come to the Mac, even PC gamers consider going back to check it out).
Excellent news for developers already working with Steam, and also good news for those of us who enjoy running our favorite games from the dock rather than the start menu. Even Apple hasn't put the kind of work into reviving Mac gaming that Valve has in just the past few months.