There is. It's because of how injection molding works. Without getting too deep in the weeds, the tool that makes the Glock frame is a big piece of steel cut in half. Each half of the tool forms 1/2 of the frame. You can see what are called parting lines all along the frame from front to back. That is where the 2 pieces of steel come together when the mold is closed. One of the principles of injection molding is that you want the thickness of the part to be fairly consistent so the plastic flows well, and fills the cavity completely. If a section changes thickness drastically, the plastic behaves oddly during the fill cycle and may give all manner of defects and sections of the tool that are not filled. This is one of the reasons they don't just fill the void with plastic. The other reason is it would take hours to cool, and as it cooled it would warp the entire part as the polymer shrinks and the part reaches design size. In order to form the closed section of the magwell, a piece of steel called a slide goes in there as the tool closes, and is removed before the tool is opened. Otherwise the part would lock the tool (you could not open it without destroying the part). Ergonomics says the length of the grip (front front to back) needs to be X, and the space needed for the magazine is less than that, so you end up with a space that needs filling somehow. The plug is a typical solution. Another solution would be to make the magazines bigger to take up the space but that has it's own tails I'm sure. I hate the unfinished look of the frame so I use them. They seem to have no negative on function, so it's just an option for those that want it.