What is the significance of the joker practicing his origin story?
Well, the joker is a very unpredictable character. Practicing his origin story could be his way of reinventing himself. Maybe he feels that his previous attempts at causing mayhem were not as effective as he wanted. By perfecting his origin story, he can create a new image for himself that is more menacing or more complex. This could also be related to his need for attention. A well - crafted origin story can draw more attention from the public, from the media, and especially from his arch - enemy Batman. He might be thinking that a more elaborate origin story will make him more of an enigma, and thus more difficult to stop.
Perhaps he wants to perfect his backstory for some devious plan. By practicing it, he can make it more believable when he uses it to manipulate others.
The joker might be practicing his origin story to create more chaos. If he has a well - rehearsed and somewhat sympathetic origin story, it can make people question their own views on good and evil, which is exactly what he thrives on. He could use this to his advantage when dealing with Batman or other adversaries. He may also be doing it for his own amusement, as he enjoys toying with people's minds.