The granularity here is entirely arbitrary. Hourly billing is by far the industry standard. That's why we default to it. It's easy to justify in court, it's easy to reason about contracts which use it.
If you're billing hourly, and the client chooses not to use your time, you don't work any hours and you don't get paid. If you then billed them for 100 hours without working, you've committed fraud.
If you're billing monthly, you've set aside the month for the client. If they choose not to use your time, you still get paid, because in your agreement you specified "use my time it or lose it".