Yup, it's a little confusing, but I think it'll make sense to you when the time comes...

The small feature you're referring to is basically just a throw-in (it's something that I can add, effectively "for free", once the rest of the code is in place). The broader feature is mostly about being able to "teleport" around large topics by focusing and then unfocusing on the posts made by a specific user (but, that's very difficult to appreciate until you've seen it in action).

For example, I often find myself in a position where I'm on one of theymos' posts in an old topic, and I'd like to jump to the next post (in that topic and if one exists) by theymos. Once you're used to that idea (being able from within a topic to one-click see just the posts made by a specific author), it becomes convenient to sometimes see just your own posts within a given topic (to refresh your memory of that topic from your own perspective, by seeing all the things you said, and who you said them to). For that particular use-case, the icon I was talking about ends up being a very natural place to hang a link (it stands to reason that if that's the icon that informs you that you've already posted in that topic, it should also be the icon that lets you discover what you've posted in that topic).