Well, it's an efficiency thing. When a layer has no effects or anything, it just draws directly to the screen, because it's just another bunch of objects. 'Force own texture' makes it draw to a separate, offscreen surface, and once the layer is drawn there, the whole surface is pasted on top of the screen. This is slightly slower since it has to copy the surface to the screen, but most video cards can do this sort of thing a few thousand times a second without breaking in to a sweat.
When the layer is drawing directly to the screen, erase doesn't work because you can't cut a hole in the screen - there's nothing underneath. When the layer is drawing to its own surface, it's cutting out an area of transparency in the surface, so when it's pasted to the screen, there's a hole in the layer and whatever was beneath shows through.
If you change the layer opacity, filter, or add layer effects, it has to draw the layer to its own surface to process the effects anyway. So you only need 'Force own texture' if you want to process an erase or mask effect, and the layer has no effects.