I think it's intended to distinguish between pants that go up to the ankles (i.e. trousers) versus shorter pants that go up to the knees like capri pants, three-quarters pants, pantaloons, hakama pants, etc.
But pants default to "long" (ankle-length) anyway, so one might as well just tag pants and that will be enough. It might potentially be useful for people searching specifically for ankle-length pants, but the long pants doesn't really get tagged so it's practically useless in this regard for searching. One might as well just search pants, which will contain mostly ankle-length pants anyway.