It is done for attention. Huge attention.
When you're that deranged and obsessed, and literary in love with the fixation, in this case the Joker, you're going to do something with that fixation makes you feel empowered and you want everybody to know about it.
Even the Joker himself does every trick in the book to get attention from people, like all psychopath's do...he does it with what he's fixated on and he wants everybody to know about it, cause it has all the means to his end.
This does speak volumes on how psychopath's operate. Joker was the means of inspiration that gave them the push to do what they wanted to do for awhile.
Their actions aren't Joker inspired...it's their sense of identity for what they are doing. They think by making themselves the Joker, they get that push psychological push, for them to commit their own fantasys of chaos and murder, which they share with the Joker.
That's my opinion on it.