The THE JOKER has been around for a long time has you may have known. In the movie he steals a helicopter and kills about 5, 6 people. Latter in the movie he drives a nice purple Lamborghini into the ocean and got a way. He also robbed about two diamond warehouses that is also why he is wanted in the city of Gotham.
If the the Joker was not in the movie the movie there would not be the hijack of the helicopter. Plus I think that is the best part of the movie and that is why the joker is in the movie. The other reason