This extension replaces BitBucket's default pull request commit message with a more useful message, especially if you squash PRs.
If your development workflow involves creating a pull request for every change, and you squash your PRs with the default commit message when you merge them, then your commit history has a bunch of entries that all look like "Merged in my-terse-branch-name (pull request #123)". These aren't especially helpful commit messages and "Merged in" just becomes noise. This extension will use the title of your PR as the commit message instead which should lead to a much more useful commit history. The bulleted list of individual commit messages and approvals will still appear in the body of the commit message.