One possible reason could be some sort of ancient bloodline connection. Maybe there was a long - forgotten ancestor who had a pact with the fox spirits, and that power has been dormant in Naruto's bloodline until it gets activated somehow. Another reason might be due to a powerful jutsu that was cast on him as a baby, a jutsu that was meant to protect him but also had the side effect of giving him the ability to turn into a fox.
Well, it could be due to a bloodline limit that was previously unknown. Naruto might have some distant ancestor with the ability to transform into a wolf, and through some circumstances, this latent ability surfaces. Or perhaps he made a deal with a powerful wolf - spirit in a mysterious forest, and as part of the deal, he gained the ability to turn into a wolf.
Speed. Wolves are known for their speed, and Naruto in wolf form could move much faster than in his normal human form. This would be a great advantage in chasing enemies or getting to a location quickly.
One possible consequence is that it could change the dynamics of his battles. He could easily surprise his enemies and gain an upper hand in combat. For example, he could sneak up on opponents and land a powerful attack without them even realizing he was there.
Maybe Naruto was trying to protect Sasuke from a certain danger. There could be a curse or a powerful enemy that targets male shinobi. So, Naruto uses a risky jutsu to transform Sasuke into a girl to keep him safe. Or it could be that Naruto found a strange ancient scroll with a transformation technique that he thought was something else and when he used it on Sasuke, it turned him into a girl.
One reason could be that he is being ostracized by the village and no one is willing to hire him for regular jobs. So he has no income source to buy food.
One reason could be that he's not getting paid enough for his ninja missions. Maybe the village is facing financial difficulties and has to cut back on mission rewards. Another possibility is that Naruto is too generous and gives away most of his money to those in need, leaving him with little for himself. Also, he might have some unexpected debts that he has to pay off, which leaves him short on funds for food.
His fox form might give him the ability to communicate with other fox - like creatures or spirits. This could be a very useful ability as it could provide him with valuable information or allies. In terms of combat, he might have a natural stealth ability in his fox form. He could blend into the environment more easily, like a real fox, and launch surprise attacks. His normal form doesn't have these specific advantages. Additionally, his fox form could have a natural resistance to certain types of attacks, like fire - based attacks if foxes in the fanfiction's lore are known to be resistant to fire.
He could turn into a T - Rex. It's one of the most iconic dinosaurs, known for its powerful jaws and great strength. Naruto as a T - Rex would be a fearsome sight with his massive size and ability to crush opponents.
One possible reason could be for a mission. Maybe the mission requires infiltrating a female - only place or getting close to a female - only group, so Naruto decides to turn himself female to better complete the task.
One idea could be Naruto having a hard time controlling the transformation at first. He might accidentally cause chaos in the village. Another could be that his friends try to reach out to the Nine - Tailed Fox within Naruto to understand it better. Maybe Naruto starts to gain new powers from the fox that are unique to this fanfic scenario.