its to release you from jinxing yourself. For instance:

say your favorite hockey team is rocking and it looks like they're going to win, but there's still 7 minutes left in 3rd period. You say, "the avs are well on their way to taking their division with this win (knock on wood)." See, if you didn't knock on wood, you would have jinxed the team and they would've lost.

:editops, rb h: