Originally Posted by
Necrodoommonkey
It seemed like a replacement for Ace Chemicals. They couldn't throw him in a vat of chemicals, so they had to show the death of Arthur and the emergence of Joker somehow. Arthur got in, and from that point on we only see Joker. We never see Arthur get out. When he's rehearsing his knock-knock joke and puts the gun to his own head, but then doesn't shoot himself, it's because Arthur is already gone. When he tells his "jokes" on the show and the audience gasps instead of cheering, it's not a delusion.
Or maybe it was just so his neighbor wouldn't hear him laughing. Or maybe it was because he used to get tied to a radiator and a fridge is the opposite of that. Or maybe it was a nod to Joe Chill. Or maybe it was some sort of coping mechanism that he uses all the time. It doesn't really matter. It could symbolize so many things that it's pointless trying to figure out what they were actually going for. This movie was heavily influenced by the Killing Joke, so it's a "multiple choice" situation.