Basically- only to clear A LOT OF weak mobs.
Their ONLY utility is buff ward in newbie stances (and, iirc, only Angie and Yega have such stuff).
But other then that - you usually want 3x Builder just to clear maps like Scar, with a weak mobs, maybe low spawnrate but huge spots.
On Steam version going 3x builder is possible way of easily clearing the BH normal in AFK mode (usually other chars needs either moving them properly due to sniper mobs, or spamming AOE skills). But... you can clear BHN on any char with decent equip, so probably it`s not the point either.
So, probably , Scar only. Other then Scar, where you need to move a lot& kill a lot of mobs (due to low spawnrate), there`re basically no place where the builders can do and,for example, 3x shotgun can`t.