Maybe there isn't an exact advantage in velocity or energy. Maybe someone just want something different or wants to use the reloading room for a new cartridge to play with, a project if you will. Maybe a shooter just wants some specific details in a cartridge necked to hold .25 cal. bullets. Maybe he/she has some spare 6.5 PRC brass laying around and decided to use it. Nowadays this can be a good reason with the lack of support from some manufacturers.
If we determined that every cartridge needed to have an advantage, we could potentially eliminate as many as 80% of the current list of cartridges including wildcats of course. Or, as some of the diehards express, there is nothing any cartridge can do that .30-06 can't in NA hunting. Insert your own choice if the .30-06 doesn't suit you.
I'm not enamored with the 6.5 PRC or the .25 caliber version right this minute. But when I start to run short of my 6.5 RSAUM necked down to .25, I can run to the PRC since there is good brass available.
Enjoy the process!