What are the 3 main functions of a budget?

Question

1. Plan
2. Control
3. ?

Answer 1

So one can live within their means.
So one can save for big ticket items to pay cash for them.
So one has the means to save for future catastrophies.

Answer 2

Flexibility.
Allowing a reasonable surplus in excess of planned expenditures for unforeseen expenditures, or savings. This is dependent on two factors.
Fixed Expenses, which must be paid.
Rent, utilities, credit payments, etc.

Discretionary Expenditures.
Entertainment, food, magazines, papers, travel, etc.
These are controllable, and should include a surplus for savings or emergencies, in any amount permitted by income.