Some questions

1. One possible reason: They're generous

2. here

Yes, but how can I give the gift, to the winner if I don't have a cd-key and it's only in my steam user?
(I think that there would be much more games here if people would get some points for giving their games)

1) Because it's very pleasantly ;D

2) If this game is presently on yours steam account and it's not a gift (e.g. your have activated steamkey, or have opened a gift or just bought it not as a gift) you can't give it to another user.

1. If the game you want to give away, is in your Steam library, then you just can't do that. Games in your Steam library are bound to your account and can not be sold/traded/given away.

2. If the game you want to give away was specifically bought as a gift (during checkout you can choose to either buy for yourself or as a gift) you can then
   open your Steam client
   -> 'Games' (4th item on the top bar)
   -> 'Manage Gifts and Guest Passes...'
   -> choose the game from all your unsent giftable games and click 'send'.
   Now you can either send it to an email address or directly through Steam to somebody from your friends list. There is no need for a CD-key.

3. If the game you want to give away is one of the few, that are activateable on Steam and you bought a retail copy of the game or you bought it from one of the other digital distributers, then you can just take the key you got from them and email it to the winner.

But, can I do this: "Right click" veiw CD key and give it, (I have for example warhammer, witch I never played with)-would this work?