Actually its more than slightly increasing the cash income already. Each level a building goes up it increases the amount it earns you per hour by more and more, so you end up getting a lot from it after several levels. I've gotten more than double the cash/hour thanks to it and with some later girls needing trillions that's quite a difference (currently at one that needs 138.1 trillion worth in gifts), and there's the second scene you unlock by reaching a certain level on a girl. Challenge mode will also be using the levels to determine the stats, so that will make it even more useful.