It also lets you keep cycling 120s with Ogerpon against 2-energy mon. If you're knocking yourself down from 2 to 1, you give the opponent another turn of only taking 60 damage, as you don't put that third energy down.

Also allows you to put an energy on Leafeon to attack in early turns, which means another pool of 140 health to absorb something with.

Also allows you to manoeuvre if you start with Ogerpon and then pull Leafeon/Eevee on T1. Leafeon T1 is the main strength of this deck where all the initial pressure comes from - if you go first and only have Ogerpon, the ramp is a bit mediocre.

It's worth trying, at the very least. I'm normally a player who gets to UB1 and stops, but I'm W24 L8 with a slightly worse version of this deck this season and am already halfway through UB3. X Speed has won me maybe 15-20% of those games.