The splinter forend makes it possible to get the lead hand closer to the barrel centerlines, which early shooters saw as an advantage to directing the shot, I suppose. (Anyone have access to what Greener or Burrard might have said about splinter forends?) Certainly, minimal forend dimensions go way back before wingshooting started. The splinter style of forend is not intended to be grasped. Rather, the barrels are grasped and the splinter forend fills the palm of the lead hand without interfering with the barrel hold. That a forend was needed at all was a consequence of gun design -- early guns (muzzle loaders) needed a forward attach point for the stock, and break action guns need a forend piece to complete the hinge. Anyway, holding the barrels directly is an unusual idea to shooters new to guns having splinter forends. It is quite comfortable and natural once you get used to it.

A shooter can point his index finger forward on any forend if he wants. Watch skeet shooters and their big weapons nowadays. So I think this is not the answer. Pointing the index finger forward certainly is not necessary to good shooting, either. Lead hand fingerpointing does occur more naturally to some when using the splinter forend, though it is a consequence to the splinter forend, not a reason for it.