The gain is negligible. It is done primarily for one reason, and that is to make the cylinder walls straight and uniform from top-to-bottom. In other words, it is to remove any "ring ridges" at the top of any cylinders, or to make any cylinders that are out-of-round round once again, or to remove deep scratches in any cylinders that cannot be removed by honing, etc.
Another reason to overbore a block is to unshroud large intake and exhaust valves, but I dare say hardly anybody here would have such a need to do so.
1979-1993 Fox Capri and Mustang VIN Decoder: foxchassis.com
Wanted (Dead or Alive): VINs, Vehicle Certification Labels, Buck Tags, Build Sheets, Window Stickers, Owner Cards, Warranty Tags, History 999 Reports, Invoices, Axle Tags