When you make a giveaway, you are supposed to gift that game to the winner (either through buying it on steam, or giving that person a key you bought elsewhere). You don't get anything in return other than an increase in the Contributor Level.

Are you new to the site? You might want to read the FAQ and Guidelines pages under the Help tab up top. Basically, when you create a giveaway and the winner of your giveaway marks the game received, you will obtain CV "points" which increase your level here on SG. The amount of CV you receive is based on the value of the game you are gifting (the price, whether it has been bundled before, etc).

The reason people like to raise their CV levels (besides feeling good about gifting games) is that it allows you to enter giveaways that are restricted to those with higher CV levels or to join groups which require a certain level of gifting in order to enter group giveaways. This increases your own odds of winning a game since fewer people are able to enter such restricted giveaways. Frequently, highly desirable games (such as new AAA releases) will be restricted to people with high CV because the gifter wants their winner to be someone active in the SG community.

(Sorry if your question was facetious and you already knew all this; tone is tricky on the internet)

You get the satisfaction of giving a game to someone else. That's why we create giveaways, to give games away to other people for them to enjoy.