Ngl I never cared for fanfictions at first because I Am Bad At Reading Long Paragraphs Unless Its Printed but I did started off reading Battle B-Daman & Crash B-Daman fanfictions because my friends in those fandom are active in writing them, and I wanted to "support" them by reading it haha. Also because Crash B-Daman is like one of the most obscure fandom on the internet so I just consume every scalp of its content
Later when I was in Kagerou Project fandom, it's the first time where I'm in a fandom where every ships have fics so that was also my first time looking up specific ship fanfics *cough* Haruka x Takane and Konoha x Ene or Shintarou *cough* This was also when FF.net was the only go-to fanfic site...good times, good times. I also started reading Japanese fanfics in Pixiv (using Rikaichan, and a lot of dictionaries) and I'd say it...definitely helped me a LOT in studying Japanese.
Also at some point where Touken Ranbu and Idolish7 was still new and had barely English fics, I continued on frequenting Pixiv fics a lot despite I'm still not the greatest at the language (but hey, again, I memorized a lot of kanji through that haha) but stopped at some point because I no longer have patience to dig through fanfics (in either language)?? Around 2016 which is also why I've stopped reading them lately. I do want to get back at some point to improve my writing skills by observing these fics' writing styles but :^)
(Also funny fact I only know that AO3 exist around 2016?? 2017?? But the...."mature" side of the contents are too much for my grandma brain I'm just a fluff-loving basic)
TL;DR: I tend to read them if my fandom or ships are really lacking in contents (because I generally don't have much patience to read them) or to support my friends; though I should probably get back to read them again when I could, it's been a while...
Hooray for long rambling again