Why does a software engineer keep old git-repo branches around, including their history? The engineer can compare the before-and-after especially as they relate to experiments, successful, and failed approaches.

A plastic surgeon might want to look at before-and-after for a few of their "branches" (specific plastic surgeries or repeated applications of a technique). "When I did celebrity-A I notice they sag too much in location-X, whereas for celebrity-B where I changed the procedure location-X looks much better." "Celebrity-P has the same odd nose Celebrity-K had ... let me consult my notes and the before/after for Celebrity-K."