What are the important things to know when quickswapping?

So I finally got Chisa and had her builtish, and now I want to learn how to quickswap. I'm gonna try and get Lucilla, but for now I have Lynae. What are some important reasons for quickswapping? What I mean by that is, for what skill would I need to quickswap for what reason that maximizes dmg? I don't want a rotation guide, I would like to have the theory with me to create rotations.