>Spending it just tells the people who make the budget that you do in fact need the amount allocated to you and to not give some of it to someone else.

Lol. An analogy of this would be a family has 10 children. Each meal, every kid is given an equal portion of food. Sometimes Timmy feels full and doesn't finish his food, so mom gives him less in the future. Because of this rule, every kid are forced to finish their food even if they're not hungry. And that's how the 10 kids got fat