I imagine it takes some time to learn to properly estimate how much to charge for a project. Until then, billing by the hour is a way to build heuristics.
No it doesn't. You simply say "I charge in 1 day increments", and the hourly-billing problem goes away. When you come to the day at the end of the project where you only use 4 hours, you have a very complicated logistical conundrum that after careful analysis and financial modeling you solve by billing the full day because you charge in 1 day increments.
When you said "No it doesn't", were you responding to my statement that "I imagine it takes some time to learn to properly estimate how much to charge for a project"? Yes, it does. However, your charge-by-the-day method is a great workaround! I'm glad you mentioned it.