So, I’m writing this really long oneshot that’s mainly about fluttershy right now, but I’m going to be making other for the rest of the main six, so this also goes for RD as well.

Okay, there’s this short scene where RD and Flutters meet when Flutters is being bullied in the high school’s bathroom. RD helps her, and they become friends over time.

Anyways, I’m trying to come up with how their friendship benefits one another in some way. For Fluttershy, RD helped her be more confident in herself and stand up for herself, but I don’t exactly know on how RD can benefit something due to their friendship.

I’ve thought Fluttershy could help RD turn down her anger, but decided to goe against that.

Any help?

Help her study.

Maybe let others in? RD is loyalty, but she's also ambition and maybe she focuses more on her numerous activities then her friends. So Fluttershy helps her to slow down a little and appreciate the parts of life that she doesn't usually get to experience in the "fast lane".

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Fluttershy could help her with sensitivity and compassion.

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how about where Rainbowdash attempts to be Fluttershy's shining armor

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That might work, thanks.

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Rainbow is always so careless that she would surely hurt herself a little in her soccer practices, Fluttershy would help her treat her wounds, and she could make lunches to her because Rainbow doesn't know how to cook
She could also get Rainbow out of the fights so she wouldn't have problems at school

I think you're forgetting something. It's likely that Fluttershy was the first friend Rainbow Dash made, since we don't know exactly when she met Gilda. Therefore, Rainbow Dash's benefit is gaining her very first friend, which could lead to her developing her sense of loyalty.

Adding to what Eddy said, Rainbow has been shown in the show to NEED admiration. She craves attention, and Fluttershy can give that to her where none else could (at the time). Also, as Justanotherstory said, Fluttershy NEEDS someone to protect her, and Rainbow NEEDS someone to protect. Fluttershy can't stand up for herself, and Rainbow's ability to stand up against great odds (even if bullies aren't world-ending villains) makes her the perfect savior.

Also, take May the Best Pet Win as an example: Rainbow goes into the episode thinking she needs a pet just like she is, but she ends the episode realizing that she NEEDED someone that could slow her down to prevent her from being reckless, to pick her up if she falls, and overall someone she could be herself around, without having to wear her "cool pony" mask 24/7. The episode pet she gets is Tank, but the Pony equivalent would be... none other than Fluttershy. Only three characters in the show allow Rainbow to remove her persona: Tank, Scoots, and Flutters. Fluttershy from their youth would allow Rainbow to show her more emotional (sappy) side, which is crucial to proper emotional growth. Rainbow would be there for Fluttershy when she cries, and in turn Fluttershy would be there for when Rainbow needs to cry.