As far as I'm concerned, the git history of a project is the root source of truth of why a change was made, at that point in time. External documentation is mostly broad strokes, API references, or out of date. Code comments need to be git blamed anyway to figure out when they were added, and probably don't exist for every little change. Pull requests associated with a given commit give the broad description of "what feature was being implemented or bug was being fixed" for a given change, but a commit message tells me what, specifically, during that work, triggered this particular change. I want to know, for example, that the reason this url gets a random value appended to it is that while implementing a new page to the site it was found that the caching service would serve out-of date versions of some iframe. It never made it out of dev testing, so it never became a full-blown bug, it wasn't the purpose of the feature branch, so it wasn't discussed in the PR. But the commit message of "Add some cache-busting to iframe" (even something that brief), can go wonders to explaining why some oddity exists.