Sweepstakes - why exact date of birth?

As a diversion, I’ve recently been entering sweepstates on SweepstakesToday. (This site is a portal to number of other sites that offer on-line sweepstakes).

On most of the contest entry forms, they have a required field for your date of birth. I can understand them wanting to know your age, so that they can send you age-related junk mailings, but why do they need to know the exact month and day?

Just a WAG, but probably they would use this to 1) ensure that you are really of legal age, and 2) to identify you in case 2 or more people with the same name enter the contest.