in gaming, basicaly because it is easier to stick based figures on the walls if one side of the ramparts lacks a wall. so the inner one tends to be left off since that gives you that ease of use without ruining the look of the exterior.
so it is purely for ease of play.
real castles usually had at least a low wall on the otherside for safety purposes, or some kind of railing. in the later situation these were often the first things to break and fall off when a castle was left unmaintained.