Seems like both "show your working" and "make it easier for us to review" are the reasons. Seems reasonable to me.

"Commit 1: refactor the $THING to enable $CAPABILITY"

"Commit 2: redirect $THING2 to communicate with $THING1"

"Commit 3: add error handling for $EdgeCase" --- long explanation in commit body

A single commit with no commentary just offloads the work to the maintainers. It's their project so their rules.