The purpose is they don't want users to leave the mouse permanently plugged in. That would leave an ugly wire to their beautiful wireless product. in my opinion the only pointing device apple ever got right was the trackpad. Their mice have been absolute crap since the iMac g3 hockey puck mouse days. During the G4 era they had that transparent pill looking mouse, then the pill with the ball scroller that would collect hair. Then magic mouse and now this. Consistently bad since the early 2000s is actually quite impressive.