You can buy it as you want it.
It does not have to be bought for everyone in your party. It can be bought when you are at the register placing your order.
If you use it to your advantage you can "save" a couple of dollars i.e. high cost entree and snack. Your snack credit can be used for butter beer, which is around $7 or a large donut around the same price. The order taker will probably offer it to you if what you are ordering as a meal will put you in that category.
My general rule of thumb used to be if my entree was in the $14 range, I would get it. I did not bother this past trip because one I have an AP I get food discounts and two, I do not drink soda, preferring to just get a free cup of water.
If I drank soda and did not have the discounting AP I would probably get it again to save a couple of bucks or if I was paying for more than just myself, but as it stands, not worth it to me anymore. I hate having to do math on vacation trying to get a slight deal. And in the best case, it is only a slight discount.