Designing a better shower battery

The one thing I don’t get with showers is why there always is water left in them. I mean, I get the physics of it, I just don’t understand why I can’t just press a little lever on the shower battery and make the remaining water flow out in the drain. Then I can be sure that no water will slowly go out of my shower and make a damp bathroom, chalk stains or grow bacteria after I’m done showering.