Sometimes maps have pimples where units get stuck all the time.
For example hovercraft often have this problem when trying to climb from water up a shore.
Or the other way around, there might be a narrow invisible path up a cliff that is not supposed to be there.
(why does this not show as image? click it)
One might want to remake the whole map but that can be cumbersome just to fix one small path?
Many maps have such pimples and fixing all heightmaps would take a long time. especially if rest of the map is "ok" that does not seem worth it.
So instead idea is to block the path with a feature, for example some rocks:
(no rocks were available for this picture, so some wreckage was used)
Such features + Lua can be added to games, so that games can "fix" the pathing as needed for its units.
It is not nessecary to cover the whole cliff with features, just the problematic pimple path.
Similiar use is to make blocked paths more visible.
For example in below picture the passage is not pathable by units, but for player that is maybe not clear at first look. Some rocks help to show that this terrain is not suitable for travel.
tl;dr:
painting face is quicker than plastic surgery