I think most of the time, the registration is there to help from getting hammered by bandwidth useage. A few however apparantly generate mail lists to sell. I came to this conclusion after I registered at one site, some time ago, and suddenly started getting a ton of spam. Okay, perhaps a coincidence. So, I grabbed a freebie email account, registered again under it, and bam, got more spam. For this reason, I am a bit more cautious now as to where I register.