Typically, and I say, typically, it is to expand the pocket to allow a full width of insulation in between the trailer uprights. Also, it allows a place to run the electrical wires so they don't have to cut the trailer uprights-they just cut the lathe to run the wiring thru an upright. Some even use 2x2s instead of lathe--

Places like in the nose, with a lot of cazy angles, you can flatten it out with shims (lathe) to reduce the number of angles of the wall board is another reason.

Another reason is that lathe (or 2x2s) can be screwed into the upright, than the wall board can be glued to the lathe which prevents screw heads showing----wouldn't recommend this, but some companies do it that way.