The map_heights.hgt is simply the heights of all the vertices. My understanding is the map is made up of tiles (I haven't really looked much into it but that's how most maps are). The dimension of the map is 255x156. Each tile subdivided you get 511x313 vertices. Hence the size of the map_heights.tga; 1 pixel per vertex. The hgt file takes all the info in desc_terrain and the map_heights.tga and turns it into an easy binary format. If however the hgt file is not needed I would guess it's just used for quicker loading purposes, and creating one would be unnessercery.