Often the sweepstakes have a long list of questions not related to the prize.
So for example an iphone sweepstake can have 10 questions asking about the total income of your household, who your energy supplier is, what kind of mobile provider you have and more questions.
Next the sweepstake owner can sell this data to companies who specialize in selling new energy contracts or mobile phone subscriptions etc.
This is not always the case, sometimes the form is short but they already make their money back by marketing other kind of offers to the leads.