If someone doesn't know what I'm talking about, they were in old client a bit like clubs but you could join one anytime by yourself just by writing the chat room's name.
I found full premades at IP weekends there and now I'd need one for Invasion. It takes so long without the chat rooms if you don't have friends who play league. Last night I spent half hour trying to find Invasion premade and still was left without a group. When I used chat rooms during last IP weekend, I found full premades in like 2 minutes. They are also a good way to find friends in league. I still have many of the IP weekend friends on my friends list.
I don't really see a reason why wouldn't Riot bring them back. Maybe people don't really use them apart from special events but they are important atleast for me during them. They may be useless for some people but there's not any drawbacks either, right?