You get points for participation on the site. Lower levels have to have their posts approved by persons at higher levels. This prevents robots, spammers and haters from ruining the site. When you reach level 8, you get a link in the notification which allows you to order a free T+Shirt, but I don't know how long they will keep that up.

To learn abotu hwo to earn points and how many points you need for each level go to your profile and scroll down to where it shows your points. Click on the points and it will take you to a page that explains it a bit more.