As Budgets serve three basic purposes.

1)Planning: It forces people to think about what should happen in their business and develop an articulated strategy.

2)Control: It tells employees what they can and cannot do.It restrains their choices and compels them to obey the plan.

3)Evaluation: It provides a benchmark.Actual results can be compared to the budget to see if the company succeeded or failed.

The question:

=> Briefly explain why a budget must be used as an evaluation tool in order for it to be effective as a control device.

My professor's really strict about word count (prefer short answer) so please provide the suggestion with just the necessary word