You get a register result that matches what actually happened
Before, you closed the register by piecing together scattered memories from the day, and any forgotten movement changed the result without anyone knowing why. With the habit of logging every movement the moment it happens, what shows up at closing matches what actually moved through the register. That gives the store a number worth trusting, instead of an estimate rebuilt from memory.
You stop losing small movements from the record
Before, you tended to leave out small amounts, like change, a quick purchase, or a few dollars going out, for seeming too minor to log. With every movement required, big or small, those amounts start counting toward the period's result along with everything else. That keeps small gaps from piling up into a large, unexplained discrepancy by the end of the month.
You give every number in the register a real reason
Before, you might log an outflow only as "expense," or not log it at all, leaving you unsure weeks later what that amount was for. With a specific reason required for every movement, the store can look back and understand exactly what happened to every dollar that came in or went out. That makes it easier to explain the register to anyone who needs to understand the business, including the owner himself, down the road.
You stop closing the register from memory
Before, you closed the register by trying to remember, at the end of the shift, everything that had happened hours earlier. With the log made at the moment of each movement, closing becomes just adding up what's already written down, with nothing to reconstruct. That cuts closing time and the risk of a missed movement.