To show a deeper connection between them. It's a simple yet effective way to visually represent their relationship. When Sakura wears Sasuke's clothes, it can signify that she is part of his world in a more intimate way.
Maybe for the sake of creating a unique and interesting visual. It can be a very appealing image for fans of the couple. The contrast between Sakura's personality and Sasuke's clothing can add a new dimension to the story. For example, if Sasuke's clothes are dark and brooding, seeing Sakura in them can create a sense of her adapting to his style, which can be used to explore her character development.
She might wear his clothes because of necessity. For example, if her own clothes got damaged or lost during an adventure. Harry's clothes would be readily available and a practical solution.
One reason could be that they are in a situation where they need to disguise Hermione as Fred for some sort of mission. So she has to wear his clothes to blend in. Another possibility is that it's a symbol of their closeness. After a particular adventure where Fred saved Hermione, she wears his clothes as a sign of gratitude and a growing bond between them.
Maybe McGee lost his own clothes somehow. So he has no choice but to borrow Tony's clothes. It could be an accident like his luggage got lost or his clothes got ruined in an incident at work.
Sakura's ability to forgive and still care for Sasuke despite his past actions could also make him proud. In many fanfictions, she represents unconditional love. This selfless love and her ability to see the good in him, even when he was at his worst, could make Sasuke proud of her as a person.
Maybe it's a dare. His friends could have dared him to wear Tony's clothes for a day and he reluctantly agrees. And as he wears them, he realizes there's more to Tony's style than he thought.
It could be for a funny or imaginative twist. Maybe to make the chicken stand out or for a special event in the story.
Maybe Sasuke sees Sakura's growth as a kunoichi. She has become strong and independent, and he admires that. So he wants her to be his.
Well, perhaps the writers feel that the Sasuke - Sakura relationship in the original was a bit forced. So they create fanfiction where Sakura doesn't love Sasuke to explore different relationship dynamics and character arcs for Sakura. For example, she could have a more platonic or even antagonistic relationship with Sasuke, which would be a fresh take on their characters.
In some fanfics, Sakura might avoid Sasuke because she's tired of his cold and distant attitude. She wants to surround herself with people who actually care about her and value her, so she chooses to stay away from Sasuke to find more positive relationships.
Well, Naruto has a very warm and persistent personality. He's always there for his friends, and he has a big heart. In fanfiction, Sakura might see these qualities in Naruto and realize his true value. He's the kind of person who can make her laugh and feel safe, which could be very appealing compared to Sasuke's more brooding nature.