Just as (or more) important, it prevents the consumer from spending money there they don't really have. For example, and a friend shared a story regarding this, if you check the gift card via phone or online after the spend, and you don't know it hasn't been processed, you are going to think you still have x amount of dollars. So then if you show up a week later at the same venue and order food, and now the previous order has finally been processed, you are told you're short, you're stuck paying for your food with other means when you thought your gift card still had sufficient funds.

So the delay in processing probably works overall to the vendor's advantage.