Willis has convinced people that he is "real life homeless man, Brad Prowley, who makes a living singing classic R&B songs on the streets of major cities not just to get by, but out of a true, life-long passion for music."
I suspect this is some sort of gambit by Willis to demonstrate that he can "act," and that he is more than just an action star. I suppose it also gives him a chance to sing, like he used to, and to be appreciated (so to speak) on his vocal merits alone.