What choose a reverse valve body?

The original purpose was to change shifting direction away from neutral for the 2-3 shift. Lousy aftermarket shifters in the bad old days would allow a 2-n shift, if you weren't careful, and it's hard to be "careful" In a racing situation. The design for a torqueflite, particularly a 727, cleaned up the shift overlap too, so the rmvb could be faster too.
There is some question if it is needed now that shifters, particularly ratchets, are much better.
The advantage of a normal pattern in a drag car, is you can kick the trans into neutral after a pass, to save load on connecting rods, and to allow more accurate plug readings, something that can't be done with a rmvb. The new (nhra legal)
shifters all have a lockout to keep you out of reverse.
For what it is worth, I have two reverse manual valve bodies, and one forward pattern manual body. I love em!