Why is snapcraft creating directories I don't need (yet)?

Working through variations of running various phases of snapcraft (pull, build, stage, prime) pursuant to prepping to some day teaching Snapcraft 101, and I noticed that, after running just “snapcraft pull”, I saw parts/ directories build/ and install/, as well as top-level directories stage/ and prime/. Why?

If I’m not running snapcraft steps that build or stage or prime, there is no value in creating those (empty) directories until they’re actually needed. Yes, I realize it doesn’t hurt, but I’m a big believer in the fundamental principle, “Do not do any work you are not being asked to do.”