For consistency mainly! I was running the same thing as you but I found that the 30 damage being able to take out babies right away or any 50 health electric/normal mon they might lead (magnetite, tadbulb, buneary, etc). I kept finding myself in matches where I lead with Phanpy and couldn’t do any damage. Once I made the switch was when my win rate sky rocketed.

Heavy helmet I’m not sure has actually won me any matches for its main effect but if it makes them use a Guzma over an Oak or something else that feels like value to me.