The time track can be used to alter the tempo, particularly useful when matching music to video hit points.
But I’m not sure how completely implemented it is–or I haven’t found out how to use it yet. I do not seem to be able to create new handles to drag, nor can I delete existing handles that seem superfluous. I admit, however, I have only used it once and have not searched out any documentation on the time track; so the instructions may be there, and all I need is to RTFM. 