While the number might be capped at 15%, it can feel like you get more whether you do or not. And some of the higher level traits really do tilt thing a little more in your favor(your tree whisperers) when RNG does side with you. Plus being at a higher level means you probly have more GP to spend on abilities(not to mention more access to cordials to bring things along faster). And i dont know about you, but i never had enough perception to cap the HQ percentage on at level items while growing my gatherers, so it feels like an even bigger gap then it probly was.