Explain Like I'm 5

We have a house listed at 850,000. I've been exploring our options in case home doesn't sell. I talked to an "investor" who was willing to give us 400,000 cash and then pay us payments for the mortgage that would stay in our name. What's the benefit for him and why can't he put the mortgage in his own name? Would we have any protection if he didn't pay us the monthly payment? (We are living in the house we are selling and will be using part as a down payment for another house)