It's simple, it brings in people who wouldn't otherwise show up at WDW. As a businessman, if you give me the choice of selling a widget for $5, or not selling it at all, I'll sell it of course!
Disney's biggest day-to-day expense in running WDW is, without a doubt, payroll. For the parks, the length of the operating day is directly tied to the payroll expenses - keep MK open for 14 hours a day instead of 7 hours, and you're roughly doubling your payroll. So when the lean months of Aug-Sept-Oct hit, what does Disney do? It can truncate the park hours, but it can't simply close the parks. And it can cut park hours, but it really can't close attractions inside the parks, or else people get royally peeved (remember when they were opening Adventureland later than the rest of MK?). Plus, I'm willing to bet Disney's labor contracts with its various unions place limits on how much Disney can cut hours and lay people off.
So if Disney has to keep the MK open for 8-9 hours a day even during the slow seasons, and it has to pay for all of those employees, then it makes sense to do what it takes to get people in the parks. Also, keep in mind that these packages are tied to staying on-site in Disney hotels. Day-trippers who stay in the Motel 6 on International Drive aren't getting these deals. You have to look at the Hotels, Theme Parks, and Restaurants as a package deal. Once you've built the hotel and staffed it, the incremental expense of adding one more guest to the hotel is miniscule - just the gas, electricity, cleaning supplies, etc, necessary to restore the room for tomorrow night's guest.
By tying the promotion to hotel stays, Disney also gets a better grasp of its numbers. They can better calculate expected food requirements, making for less wasted food and more efficient operation of restaurants. If you're staying off-property and buy your tickets at the gate, Disney has no advanced warning whatsoever of your arrival, so they have to keep a little reserve capacity everywhere to accomodate that.
This all just makes your realize how much money Disney is making in the summer months, where they have jam-packed parks and hotels, and very limited perks!