It's just for compatibility with the P16PRO40, so that you can program any size PIC in a single 40 pin ZIF socket - you need to have two different Vpp switching circuits. Vpp40 is used for 40 and 28 pin devices, and Vpp for all the rest - it prevents you putting 13V onto pins that you shouldn't!.
If using WinPicProg with a programmer that only uses one Vpp switching line, you simply set them both to use the same port pin.