Because it helped in the slow death of fandom forums.

Back in the days, you went to Johnny’s Anime Forum. Then people demanded Johnny to give them the same freedom 4chan allowed. Johnny either bade his forum hostile to others, or let people go to 4chan.

Yes, the ultimate blow was Facebook groups and pages, alongside with Reddit, but 4chan was the first blow.

Nowadays the closest thing we have for that is Lemmy/Piefed/mbin.