Budgets are used for both planning and monitoring or controlling. Your initial budget is set up to get an idea of the project. For example, during the planning stage, you set you targets and you get an understanding of the resources (human resources and funding for example).
However, it is also important to benchmark the budget you created when you planned the project versus the actual. Doing this exercise will you understand how well you are doing and what are the things that you should be working on.
The budget is not a “develop and forget” exercise. It’s a tool that you create to understand the project you are about to undertake and also monitor the project and make it better.