Batman's real name is Bruce Wayne, chairman of Wayne Enterprise Group and the richest man in the world. It was in one of his restaurants that he first met Harvey Dent. Prosecutor gotham city wanted to date his colleague's fiancee Rachel, so he asked someone to reserve a seat in this restaurant. He happened to meet Bruce Wayne, who was holding a Russian ballet dancer. Harvey Dent, who didn't know that Bruce Wayne was Batman, talked about Batman and his words were full of worship. He thinks Batman is a hero, this chaotic city needs Batman's care, and he is worried that Batman is under too much pressure. "Either he died like a hero, or he lived to see that he was forced to be a bad person."
Harvey Dent was Bruce Wayne's rival in love, but Wayne was moved by Harvey Dent. He wants to use his wealth to help the "Knight of Light" and make Gotham citizens "trust Harvey Dent (this is Harvey Dent's campaign slogan)". He also longed for Harvey Dent's wish to come true and entrusted him with the responsibility of "caring for Gotham".
Of course, there is great selfishness behind this selflessness. Because Bruce Wayne still loves Rachel, and Rachel doesn't want to marry Batman and live a life of fear. She wants to live a plain and happy life with Bruce Wayne, so she told Bruce Wayne before that if you stop being Batman, I will marry me.
So "Night Knight" wants to give the burden of taking care of gotham city to "Light Knight" so that he and his beloved can live a happy life.