What are MTRs used for?
There is a description of this route page 1-139 of "Area Planning, MTR". 

  

The scan is not good, you may get the textual content here too.

It says terrain following activities may take place, as well as visual contours (which I assume to be navigating without radar, looking at the outdoor and searching in a catalog the same relief contour to know where you are). As you commented, taking into account the local relief:


For terrain following should be for newbies.
For visual contours navigation should be for top-guns only.


There are different routes like this, starting from nowhere and ending in the Smoky MOA. They seem to be routes avoiding nearby airfields. On the image below, I've emphasized the area without airfields, including MOAs. It looks like IR-504 is drawn along the centerline.


Based on VFRMAP

Other possible selection criteria:


Unpopulated areas (noise from low flying jets)
Low radio noise areas.
Flat terrain for UAV training.