While it looks like this was answered for the most part I thought I'd go ahead and throw an example in with some numbers.

ALB has runway 1/19. I forget which runway it is but one of them has obstacles that put a significant hit on my max t/o weight. For easy math lets say that t/o weight for 1 is 50,000 and for 19 is 45,000 (ALB does have a few thousand lbs diff). I'm allowed to take off with up to 10 kts of tailwind. I have to take a hit for each kt of tailwind..for easy math lets say 200 lbs per kt. So for a 10 kts tailwind I have to take a 2000 lb hit so subtract 2000 from 50,000 and I can take off runway 1 with 48,000lbs compared to 45,000lbs on 19 so in order to take the extra bags or people we will takeoff with the tailwind. We do get a "credit" for headwind, but it usually is only a handful of pounds per kt and usually doesn't do much good, unless its pretty windy.