In my experience, it's rarely about flagrantly wasting money for the sake of running up expenses to a budget level. Rather, it's usually more about pulling in expenses, doing something you had wanted to do anyway but didn't think you had enough money for, etc.

Budgets are a pretty powerful and widely used tool for managing organizations. And there are other checks and balances. It's not like a manager can necessarily just decide to have a team off-site in Hawaii because there's room left in the budget.