A well developed budget is accompanied by a budget explanation. A complete and realistic budget justification demonstrates that your project is well conceived. It also tends to minimize the chances that sponsors will arbitrarily reduce or eliminate budget categories. Sponsors have a good idea of what a project should cost, and generally know when you are over or under budgeting.
The budget is reviewed by OGCA and the sponsor to verify that costs are reasonable, allowable, allocable and necessary to carry out the proposed project, and if it conforms to the sponsor's instructions. During award negotiations a budget is sometimes subjected to further analysis by the sponsor's audit staff. Thus it is important to maintain all the documentation and justification you can assemble for each cost element and category, in case the sponsor questions items and estimates.
For newcomers to the process, consult with experienced faculty or staff within your department for advice, or contact the OGCA proposal team to discuss potential costing.