Since your deck will be full of instants and sorceries anyway, that prowess ability can grow it into a 4/5, 5/6, or something waaay bigger depending on how many cards you can play in a turn
This means you could drop a land and a few Mox, then trade your hand in for a brand new one on your first turn, and leaving your opponent with a potentially bad hand.
Some character and item cards also have special abilities that you can use on your turn
Honestly, I can't tell if it has been this way for a few years and I'm only now noticing this trend, but I certainly feel this has been the year that showcased this duality most of all
As a result, Atraxa provides a perfect backup plan in Legacy Elves
But the starter deck itself only comes with one copy of each of these cards