It seems to be a shortcut to open the comment url. Specifically opens the full article but at the height of the comment from which the link was generated, by pressing the icon.
There is a bug and the links are not being generated correctly, because they lack the part of the url that refers to the publication in which the comment has been made.
I will now make a comment to have a practical example.
Oh, that's not exactly explained by the UI. It's a feature not common to other websites, even mastodon.
I agree, don't remember how I discovered that, but it sure comes in handy when participating from Friendica !