I use Kongregate loads, and for two main reasons:
There are always really good games, from the newest (e.g Learn to Fly 2) to the classics (Age of War <3)
Badges. They use an ingenious achievement system, where you perform feats in the game, and get badges and points for doing it. These points then add up, and are represented by a level.
I find it's a really good way to motivate you to play a game and achieve certain tasks, and adds a feeling of accomplishment to the game. Also, since Kongregate is part of the GameStop network, there is talk of future rewards based on points earned, such as Xbox games, gift cards, etc.
You regularly get challenges that have added bonuses, such as an entry into some sweepstakes, or extra points, or a new avatar and quest.
Overall, I think it is a really fun gaming site, that I visit regularly!