Why is Rent Due on the First of the Month?

[Julie Marquez]

So this may sound like a basic questions, but why is rent due on the first of the month? What about people who want to move during the middle of the month? Other bills in life are billed monthly, but the due date is based on the start date - like my car payment due on the 25th of each month. Has anyone every done a system of rents rolling in based on when they moved in? Or does this have to do with move out notice laws/procedures, like 20 days notice before the end of the month? Is this just the simplest way?