1. setting a minimum level is often used to only get players who are activly playing.
getting small lvl members often means you have to kick them for inactivity, and start looking for new ones

2. the number of girls you have is a indicator for your activity and to see if you are f2p or a koban user
besides, the more girls you have the more money you make, the easier is it to get stronger

3. once you maxed everything in the club there is (currently) no more to upgrade/donate. the bonuses are maxed out