The Chida says to use every step of the Seder.....when you announce a"Kadesh" that you're going to you have a Kavana or Tefillah that no anger should come during this step. Because you're susceptible to anger. And part of the the miracle of Makos B'choros" is that Vilo Yiten Hamashchis Lavo" , there was no opportunity for the Mashchis to come into the house of the Jews. So you're trying to protect yourself primarily against this Mashchis" who's really fighting to get into the