Do you live alone? If more than one person runs the dishwasher in your house, then you have to go off of how the dishes look. Many dishes don’t look visibly dirty after using them. You can root around in there, looking at them all, and hope that you make the right judgement call (otherwise you’re putting away dirty dishes or adding a dirty dish to the clean dishes) or you can just glance at the detergent reservoir and know that closed=dirty and open=clean. It takes no more time than any other way of doing the dishes because you still have to fill up the detergent at some point.