Perhaps Dumbledore saw a future danger that could only be averted if Harry was a girl. It could also be related to some ancient prophecy that is linked to a female - form Harry. Or it could be a test of Harry's adaptability and his ability to handle unexpected changes in his identity.
Maybe Dumbledore wanted to protect Harry from something specific. By changing his gender, he could hide Harry from certain threats. It could also be a way to teach Harry something, like seeing the wizarding world from a different perspective.
One reason could be for a strategic mission. Dumbledore might think that as a girl, Harry could get closer to certain people or places that are only accessible to females. Another possibility is that it's part of a magical experiment to understand the nature of magic and gender transformation better.
Chapter 906, "Big Brother, are you jealous?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on A few people were somewhat surprised.
If you want the clearest chapter reference, go to Chapter 24. Early on, Not only that, even the non-mainstream photos she had previously posted on...; later, the chapter leans on Phrases such as "ugly girl craving swan meat," "arrogant, wasteful....
If you want the clearest chapter reference, go to Chapter 32. Early on, You should know, an ID photo is like a mirror for demons.; later, the chapter leans on Even celebrities can't handle going barefaced under the attack of a....
Chapter 894, "I Want to Attend the Same University as My Sister", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. It opens with The sound of early summer cicadas was close to the ear.
Chapter 905, "I Plan to Quit the Circle", is the clearest early answer because it turns Jiang Bufan's situation into a concrete plot event.
Chapter 897, "Are You Worried About Me?", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter turns that into something concrete by moving from Changqing stood at the roadside with the night breeze blowing,...
If you want the clearest chapter reference, go to Chapter 13. Early on, Sensing that something was off with Jiang Yan, Jiang Yan dismissed the...; later, the chapter leans on What on earth do you mean?" Ignoring him once again, Jiang Li shifted her....
If you want the clearest chapter reference, go to Chapter 8. Early on, The resentment that had barely calmed among the crowd surged back two...; later, the chapter leans on Some of them were relatives of the Jiang Family, who felt it improper to....
Chapter 885, "Isn't This Wine?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Daoist Shuiyun's words weighed heavily on Jiang Li's heart like a.... By the end, Even replacing all the tea in his room with wine..
The best chapter pointer here is Chapter 13, "Killing the Chicken to Scare the Monkey". It opens with Sensing that something was off with Jiang Yan, Jiang Yan dismissed the... and closes on How could this happen..." Lin Manru didn't want to hear any more of her....