Really think it happens because of the extra ice time. You're in the game more. Engaged in battles and making plays. When you don't get PP or PK time as a forward, you will be limited to somewhere between 10-12 minutes per game. Also, if a coach trusts you on the PK, he also trusts you at the end of games. All of that builds confidence.
I'd argue that I want Lafreniere and Kakko to both be part of the PK. Imagine Lafreniere or Kakko with Trocheck killing penalties. On top of the fact they can make the plays in their own zone, they're liable to go the other way and create chances. Carpenter can fill in if needed, but rolling something like:
Zibanejad-Kreider, Trocheck-Kakko, Goodrow-Lafreniere, you're talking about a dangerous unit. Then you could roll something like Panarin-Chytil-Kravtsov, which could be a great line to go out there and take on a team's 3rd/4th line forwards after a PK.