Saves me resources. I was unprepared for cyan+3 - usually I save up more stamina packs and diamonds, but this time I didn't. However, I still was able to farm all the items I need when I eventually hit 145, plus a few that I won't need until cyan+4 or later, because well over half the heroes I use don't need to be promoted. I have many heroes still at cyan and cyan+1. It also gives me an advantage over people that just promote all their heroes to the latest rarity.