but why

Couple of big things stand out to me. His possession numbers are sparkling. I won't go into every detail, but when your Relative Corsi For is positive, has been almost every year you've been in the league and you play for a high possession team like the Flames, you're helping to drive play, which is something the Isles' third line could use. Besides that, his entry and exit numbers are solid, better than anything Komarov put up last season, which is who he'd been replacing. I know both of those points are analytics based, so if you just look at his simpler stats, he was on pace for about 40-45 points, which would've put him 5th on the Islanders point leaderboards from last season. He isn't really that much different from Filppula or Komarov (he's actually much better than Komarov), so having him in the lineup shouldn't be too tough a pill to swallow.

TL,DR: Frolik is a good possession player, who scores a decent amount, something the Islanders don't have a lot of