Tempo and/or key changes.
Mostly, I use them for hit points while scoring video/film.
Also, using the markers when editing dialog or a monologue is a great idea. Man, especially with large (long) projects. Most of the time, it is pretty easy to follow music/song structures when looking at the wave form, but spoken words can be tricky. Labeling the markers with a character name or a few words from the dialog usually helps, too.
Brad