“Why does it matter to Trump if he is popular with Americans now?”
that is the only thing that matters to him. What drives him in life is “winning” and being on top. This is why he brings up his Greatest Electoral college victory” (IT wasn’t the greatest but fcts dont matter to him). It drives him bananas that he lost the popular vote
“What’s the point of having a pep rally when he’s already been elected?”
Se above. Money doesnt drive him, it is just a way to keep score (as he wrote in one of his books). she once asked him if he would want to be appointed PResident, he replied that running/campaigning was the main point
“How does that change anything?”
See above
“Is he focused on winning in four years now?”
Not necessarily
“Does this do anything for the country besides make him feel better about himself and/or convince people that he is right?”
It only does the former, nobody will become convinced that he is right