IMO, Trump would be doing the rallies and raising money regardless of a 2020 decision. He needs to do that to keep pressure on Congress to enact the will of the people who voted for him. These rallies also serve as a continuing reminder of his popularity which makes it more difficult for a rogue prosecutor to put forth some weak, but damaging charge against him/his administration. Of course, Trump isn't a quitter, but there are plenty of other angles the future may take. For example, Trump will have tremendous power as a king maker in 2020 if he decides hand off the baton to someone else. Personally, I'd like to see him in office for 8 years because the swamp is deeeeeep.
Lots of reasons for the rallies but I think he has the time of his life and just loves doing them..