i think the idea from the minimum effort drill is twofold:
- to understand how little effort is needed for specific tasks: getting the ball over the net, creating topspin, hitting near the baseline, serving.
- to have smooth movement, which gives you an understanding of how kinetic energy can be transferred from one body part movement to another
these points allow you to Calibrate your effort to what you want to achieve in a particular shot. You can better manage your effort over a match and have improved feel as you're better able to control your power, i.e. you can hit the same shot at different paces.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM