They were huge in the 90s because it was the first time basically anyone could create such a thing and distribute it freely online.
It was a creative outlet for fans who wanted to express their love/appreciation for whatever anime/show they were into, and it also served somewhat as a marketing tool for the shows themselves.
It doesn't seem like anything impressive anymore since we all have access to more advanced editing tools and bandwidth and online engagement is done by everyone instead of just geeks