>You don't end up paying less.

Correct. Your costs are smoothed over a 12-month window so that you pay the same amount each month.

> They just charge you what they think you'll average out to have. Then if you're over you'll get a huge bill later.

You don't get a huge bill later. It rebalances once per year and your rate will go up or down based on prior usage. The idea of "budget" billing is less about reducing your bill and is more about making your bill the same every month so you can count on it as a fixed expense.