Upon death, Snow Poff summons a Greater Dog to the board, one of the best Taunt monsters in the game. The Greater Dog will be summoned to the board right away, so you don't have to pay for it. However, Snow Poff cannot attack itself and doesn't have Taunt, so it can be avoided until they can get rid of it easily with a silencer.
Best used with Worsening (in Perseverance), Punishment (in Justice) or Vulkin (in general) to instantly kill it yourself. Giving it taunt using Protection (in Patience) or Glad Dummy (in general) also works well.
If you leave it on the board, you have a high change it gets Silenced before easily being killed, so just like with cards like Mettaton NEO, kill it yourself right away.