Personally I'm expecting them to try and make a profit out of this by selling extra items and maps etc. I'm guessing that's tied into it.
I'll bet you money that they won't.
It's not a big mystery why they did this. Valve releases quality free game, BUT only on Steam. The distribution platform they own. People not already on Steam, of which there are millions, will get Steam to get quality free game. Now Steam has a bigger userbase, and the newcomers will potentially stay on and buy other games. It's advertising plain and simple, except in this case, the advertisement is a quality free game.
It's no different than Old Spice investing money into their latest series of ads. Heck, I bet the budget allotment was no different too.
As to why they chose this game? That's another simple answer. Alien Swarm is already free. As a mod. In fact, as I hear tell, it's actually BETTER with more stuff on the mod. Just not as pretty. The amount of profit they would have made trying to market this game (admit it, you wouldn't have cared as much if it wasn't free) is insignificant compared to the publicity gained from this stunt.
You know who are the people I don't understand at all? The ones who want to pay real money for this free game. What is wrong with those people? Do they attempt to pay their grandparents when they get presents? Are they rolling in money or something that they want to throw it at things? Give it to charity, geez. Hell, give it to me!