I think Trump likes Christie if for no other reason that he puts NJ in play for the GOP. It has been leaning away from Dems for the last few years. If he can flip NJ it offsets risk of loss from PA or FL. Not entirely but helps. Dems need NJ more than GOP does, and worst case it forces them to allocate resources to keep it all at minimal cost to the GOP since any support gained will be due to Christie and not due to outspending the dems.
Christy is NOT popular any more in NJ, he would NOT win today if running!!!