If you ask NHL GMs, it will usually be some variety of the vet has the experience of being in games, so in roles that require you to basically play even with the other team (fourth lines are a success if they eat overtime and don’t get caved in), having a guy who’s been in games with NHL talent has a better chance at being steady Eddie and the kid (with waiver ability) can go down to the AHL, play minutes commensurate with what you expect of them out of the spotlight and if a vet gets hurt, you’ve got the kid in reserve.
It’s designed on organizational depth more than only caring about the NHL roster.
Plus, for fans, they’ll say “Motte will screw up too, Mazur might screw up but he’s a kid and that’s the learning process.” Whereas GMs (Holland and Yzerman specifically) have been big on pass/fail. That the NHL isn’t a league to develop people in, it’s a league where you should be attempting to win every night.
Now, Johansson should be on the roster because he’s out of waiver eligibility. But Mazur, unless he shows out like Raymond in his promotion year, should start in the A