In some projects JIRA issues can be large, complicated and have a long life. In such issues you may need to review last comments to recall its context of the task.
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.
Personal notes dashboard
To access dashboard open personal menu
Select “Personal notes dashboard“. On loaded page you can review all notes you made, make necessary updates or assign personal due dates. You can load particular set of issues (current sprint or you popular issue filter). Combining criteria with “With my notes only“ checkbox you can add new notes.