Is it like iTunes for FPS games?
Pretty much, yes. Specifically games that people who are hard core into FPS consider to be "vintage" or "classic" - even though said games are many years younger than Wolfenstein, Doom, or even Quake. People who are big fans of it see it as a great gift to be able to buy Half-Life for $8 even though they bought it for $40 the first time, then bought the first special edition of it the year after for another $40, and the uber-mega-titanium-coated-diamond-edition of it the third year for another $60.
For the most part, it is a way for a company to extract a little more money from a large number of gamers who like to keep buying the same thing repeatedly.
And yes, I know I will be lucky to not be moderated down for this. Bombs away.