Understanding a diff usually requires knowing and remembering the codebase well, having a very clear code, the diff being reasonably short and atomic and time.

Understanding a diff usually requires knowing and remembering the codebase well, having a very clear code, the diff being reasonably short and atomic and time. Commit messages, if good, are extremely helpful to check what changed e.g. in an open-source application you're using. Of course the commit messages can lie, so you should also check the diffs, but it's much easier to confirm what a diff does than deducing it. Not to mention that for many changes you can only make a guess at what the intention was; if you have a message saying what was meant to be done, you can check that it really was done correctly.