If you know exactly what your opponent is playing, you know that getting the first attack in is crucial to winning the matchup, and you’re confident you won’t whiff the first turn attack, then you should choose to go second.
But most of the time those conditions won’t all be met, so you’ll want to go first. It’ll put you one turn ahead of your opponent in terms of general setup – one energy attachment ahead, one supporter ahead, and so on. It’s advantageous. And this is without even considering the possibility of your opponent locking you out of items by way of a first turn Trevenant or Vileplume.