You might get some flavor, some say you can get a little more IBUs. But at mash temp the isomorization reaction will be slow, and you might get 1 IBU? It might help with the run off. I have done it as in the old Pliny the Elder recipe that had mash hops, and I did it without the mash hops. With all of the other hops in there, no difference was noticed in the beers.
I think it is a way to use more hops, so one can say my beer was even mashed hopped to get all that hoppy goodness in to the beer. Makes one feel superior. Until you hear about adding hops to the sparge water. Then you feel inadequate.