The way to download a video was hard enough with the crappy download speeds, and not to mention the places where you could actually find porn, without having to pay for the actual thing. When talking about porn, these forums were probably one of the first things that evolved when the Internet porn started to take over the whole business. People still to this day have the urge to talk about the topics that are relevant to them.
Over time, this changed but the forums remained. Forums, where you meet other like minded peopleEver since the ancient times people had to gather somewhere and chat, talk, debate and politicize about relevant topics.