Having a clear budget puts you in control of how things go. Once you know how much a day of filming will cost you on a particular production, you're less likely to get caught out on extra expenses or worse have to cancel or change because you didn't plan well enough or early enough.
I am terrible at maths. I still can't recite my times tables properly so the idea of 'budgeting' always scared me. Until I had to do it in university and realised that it wasn't so bad and that it had it's perks.
I've tried to explain how to do a simple budget in this video. If you're just starting out, try with a short film first, to get the hang of it before you move on to a large production.