This quote from DocGKR pretty much sums up why you might want it:

Because of the larger permanent cavity and greater bullet mass, the 7.62 x 39 mm JSP’s offer somewhat better performance than the .223 bonded JSP’s, like the Trophy Bonded Bearclaw use in the Federal Tactical loads. These 7.62 x 39 mm JSP loads are a good choice for use against car windows and should also be outstanding for hunting deer and other similar size game.
Cheap blasting ammo and the potential for better terminal performance. If you wanted an AR for blasting and hunting it's pretty easy to see why a 7.62 is attractive.