It also helps Valve keep excitement for half-life 2 going strong when they choose to release episode 3. Then their story won't be "Hey, remember when we released the last chapter to this one back in 2007? Well go play the new one!" Instead, they can say "Hey, remember Half-Life 2? Now you can play it all over again and reacquaint yourself! Then you'll remember why you want episode 3 so bad!"