The option.tmc file replaced the repaint.tmr a while ago. It can define the name of a livery but you can also attach new parts to the aircraft available just with that repaint.
We use this to add new wing tips in the a320, the low skids in the ec135 but you can really get creative with this feature. You could have external loads (that physically add mass to the plane as well as aerodynamic forces), different tire sizes, different landing gear options like skids, floats, wheels, tricycle, tail wheel, 3 bladed prop vs 2 bladed prop, new landing light color (e.g. brighter LED lights), different panel configuration or colors....