They want your name and address for marketing. There's no other benefit other than what UD mentioned - some can't be bothered or they forget. Also, the company that offers the rebates generally isn't the company that you get them from. There are special rebate companies that do this and it's no skin off the original company's nose if you don't collect or wait six months for the money. The only rebate worth getting is the instant rebate which usually has no strings attached.

Bryan