The purpose of a budget is to use good judgement in order to pay your bills and budget your money. Good judgement is mostly important because you need to be able to use good judgement to determine what bills are "needs" and what bills are "wants." This makes it so you take care of everything. The other reason I would put, is to be sure you can order your bills from most important to least important. So that you can take care of the most important "needs" before the less important "needs." Like you might need a phone, but having the unlimited plan might be a want, so the phone bill wouldn't be as important as the electric bill, even though both are needs.
