They do it for fame and recognition mostly. After gaining access to a site they usually deposit a file on the server. They then post their success at a site that validates their claim by checking for the presence of the file. Once validated they are a awarded points. It's every scriptkiddies dream to get on the 'top ten scriptkiddies' list.
This is the most common reason for hacks. Usually they are just kids who downloaded a vulnerability scanner of some sort, read a how-to and hope one day to make a name for themselves on the kiddy scene. The real 'crackers', who are after government/corporate secrets or are into credit card and identity theft don't bother with our small time sites.