Blais was a member of the Blues when they won the Stanley Cup in 2019, and Berube was their coach at the time. They both have Championship experience and the Leafs could sure use some of that as they continue to fail year after year in the playoffs.
The Leafs already do have many internal options on their roster for the upcoming season, however seeing as a player like Blais who has won the Stanley Cup and played for Berube is still available, it wouldn't hurt to try and bring in that experience and physicality, as we've seen those two things matter a ton when games get serious in the playoffs.