Changeset comments are useful to allow others reviewing change set to understand what happened in the commit and promotes trust. They are also just community consensus best practice. All commits are public so its good to have good descriptions for what each commit for the same reason change set comments are used in other VCS’s, allowing for changesets to be easier found when trying to track down an issue and when it was introduced. For more information please see https://wiki.openstreetmap.org/wiki/Good_changeset_comments. 