Carrying helps with maintaining possession of the ball just after a catch is made. If you watch replays, you'll notice the receiver occasionally bobbles the ball. This is also presumed to be the point at which the Jackhammer VA takes effect.
you are right I have noticed this a lot with my guy. He makes the catch but seems to bobble it at times. This gives the defender time to get to him and it really cuts down on his YAC.
Many people underestimate carrying for RB too. Most of the performant RBs have a high carrying (at least 2nd capped). Some people say that it helps elusive backs use SAs/make moves too.
I'm wondering if this is working too for speedster WRs and their dedicated SAs tree.