You maximize your traits by earning Hunie through conversation. Once you have maximized your traits is there anything you can use the Hunie you continue to accumulate?

Not sure if you need to bed all the girls first, but you should be able to use it to buy specific gifts for the girls that you haven't given them yet. You'll have to buy through the individual girl's information screen, the one where you can see which gifts you've given so far.

Mostly a 100% completion thing.