The estimating is often enough to make better choices.

I know I’m not going to be able to eat my main, a couple slices of pizza, one or two entrees and a dessert with only 800 calories left in my budget.

Sure, I might be somewhat off in my estimate, but in practice, I might forgo the entrees and dessert (or share a bite from someone else), set some of my main aside to take home, and have a slice of pizza.