In some projects JIRA issues may be large, complicated and have a long life. In such an issue you may need to review last comments to recall its context.
Using Personal Notes add-on you can store the "next step" that you need to do to move the issue.
Your "next step" may be noticeable for you but absolutely useless for others, that's why it's private. If your note is important for others, it's better to use comment to share it.