What does " X miles in trail, runway 25R cleared to land" mean? I was listening to the LiveATC Phoenix North Tower feed as he was clearing aircraft to land. He said something like "Southwest fifteen sixty five, Phoenix Tower, (aircraft type), X miles in trail, runway 25R, cleared to land".

What does the "(aircraft type) X miles in trail" portion mean?

Is the controller pointing out the aircraft behind the aircraft that is being given the landing clearance? If yes, why would the aircraft in front care?
The controller's telling Southwest 1565 it's cleared to land (otherwise they would use the other aircraft's identification), and by the way there is an airplane of a certain type coming up behind and to take that into consideration in the event that Southwest 1565 was thinking of spending too much time on the runway after landing instead of promptly getting out of the way.  

That kind of comment added to the landing clearance suggests that the airplane behind is a little closer than usual, or is going significantly faster, or it's just really busy with airplanes spaced at minimum separation, and Southwest should expedite clearing the runway.