The advantage for the Canadiens is that Mantha will become an unrestricted free agent at the end of the current season. The advantage for the Capitals is that they could save $2 million on their salary cap this year, but they would still have the hefty $3.4 million contract of the Finnish player next year.
It's true that he hasn't produced at the level expected from a $5.7 million contract, especially since he hasn't recorded any points this year. Nevertheless, Kent Hughes has the cap space for his contract, and he would have one less issue to worry about for the following year.