In Fellini's 8 1/2, the director Quito is creatively stuck while trying to find an ending for his latest film. He sets out on a path of 'retiring' the memories and moments of his life that have defined him and his creative spirit. At the climax of the film, he realizes he can integrate all those memories to support his creative self and this is acted out by a celebratory ending that includes a marching band. This scene has been recreated in other movies, including Big Fish and Shortbus.
The moment of celebration in Joker is set off by Author Fleck killing a talk show host on live TV setting off a riot for which he is a symbol. But the intense focus on Arthur's life never quite explains why he has become that symbol and, if he is that symbol, why anyone would care about Arthur Fleck.
Somehow the moment is meant to convey that Arthur is fully integrating with his persona the Joker. I was left wondering if it was just another delusion.