"My understanding,which could be wrong,the forend cap was to seal off the end grain of the forend."
Further, that ebony is quite a bit harder than the walnut, and the tip is probably more likely to bonk stuff. Probably the same reason the grip pommel is usually something harder. The Schaubel stag forends (which I think look like mushrooms or leg bones
) are also a way of armoring the rifle, while giving a nice hand stop. As old as the practice is, I highly doubt it is purely decorative in function (styles come and go); these rifles were purpose built as lightweight and durable game stalkers.
TCB