It turns out that TrackBack was initially developed by Movable Type, the blogging software developers.
In a nutshell, TrackBack was designed to provide a method of notification between websites: it is a method of person A saying to person B, "This is something you may be interested in." To do that, person A sends a TrackBack ping to person B.
They also say ...
Although TrackBack's most prevalent use thus far has been as a form of remote commenting, a more exciting use has been emerging: using TrackBack to aggregate content into topic-based repositories. This was actually the original intended use of TrackBack--the remote commenting grew out of a special case of a topic-based repository, the "topic" being a single weblog post.
I can understand that in theory - one way links are a great shortcoming of the web - but it seems to me that - at least for the moment - apart from rare occasions, where one might need to notify many sites at once of a relevant post, the complexity of using trackback rather than a simple comment will render it a tool for a minority.