An unexciting idea: Code changes have context
I recently read Mark Dominus's I wish people would stop insisting that Git branches are nothing but refs (via). One of my thoughts afterward is that this feels like an instance of a broader thing, which is that (code) changes have context; here, one part of that context is where they happen (ie, what branch they happen on). Of course we already know that in a sense, because Git (and pretty much every other version control system) considers it important to record both who made the change and when it was made.
In a way, it is turtles all of the way down. It's not too wrong to say that in Git, the core objects are trees. Changes (commits) are a record of the relationship between trees; they give you the context of moving from one tree to another (often partially literally in the form of the commit message and anything it points you to). Our desire for this context is one reason people emphasize that you should write good commit messages. In a sense, diffs themselves are an expression of that context, since they are literally what changed between the two (or more) trees involved (although diffs by themselves aren't necessarily enough context).
When we move one more level up, branches are one expression of the context of changes (commits) themselves. Branches generally have some sort of meaning, and they also represent (are) separate sequences of changes; that separation adds context to the changes themselves, although for more context you need to know what the branches are. Of course branches aren't the only way of adding context to changes (there are many ways of putting it into commit messages). Nor are they the only context to changes we care about, since sometimes we care if particular changes are in a release or in a version that someone is running.
(The question of 'has this change been merged into the main branch' is an interesting edge case. Here, we do care about the state of a change in the context of a branch, but it's not the branch the change was initially created in. Knowing that the whole branch was merged into the main branch would only be helpful if you knew that the branch didn't continue on beyond that merge.)
A corollary to this is that you'll forget this context over time. This makes me feel that it's worth putting as much of it as possible in a durable and accessible form, which probably means the commit message (since that's often the most accessible place). Code comments can help, but they're only attached to the new state so it may take some contortions to discuss the change. I've sometimes engaged in this when I think it's important enough (or where I may not think to find and look back at a commit message), but putting dates and discussions of how the old state used to be in comments feels somewhat wrong.
(I suspect that all of this is obvious, but Mark Dominus's article crystalized this in my mind so I feel like writing it down.)