Back in the days of poorly made geocities and angelfire websites, with bbs to communicate and ftp to transfer files, irc was an easy way to connect with people with shared interests and talk in realtime.
Chat sites like AOL Chat took some of the appeal away, but only in tendem with more sophisticated forums that supplanted irc as a mode of communication. This is no different than what Reddit has done to forums, really, although it's limitations have become more realized within the past year.
The advantage of IRC today is still talking realtime with folks, but without the need or complication of the social media aspect (adding people to friends/social circles, etc). Just connect, join a room, and talk.