Despite the years he had spent in the Realms and how much he had changed, Danny could admit that he missed being a hero, or at least a vigilante of sorts. Being a King was stressful and everyone asked for his opinion, something that had improved over the years but had never completely changed.
So when Clockwork told him that there was a hero named "Batman" in another dimension, he was interested. Mainly because the people there seemed to think Batman was a ghost or connected to the supernatural, when in reality he was just an ordinary person.
Sure, he was also a billionaire but everyone had their flaws and Danny didn't plan to hold that against him. Someday he would meet a normal billionaire with no hidden agendas, he was sure.
Batman had children too, even though he "worked alone" and the halfa was very interested. So much so, that when his mentor asked him if he wanted to spend a vacation in Gotham pretending to be the vigilante he agreed without much trouble.
It seemed that Batman had been badly hurt and people were starting to lose their fear of him, something that was critical in Gotham. So Danny agreed to his mentor's request and imitated the Bat's costume as best he could, adding a more supernatural effect than usual.
It wasn't long before Bruce found out about his double. A double who was doing his job very well and scaring his villains while he remained in bed, he would be grateful if it wasn't so worrisome.