The benefit, though, is once you are set up with IRC, joining new channels is a breeze, especially since most open source software uses a single network (Freenode). All I have to do is type something like

  /join #rust-lang

and I'm in the new channel. Compare this with Slack, where, no matter how many teams you've joined in the past, you have to go through the exact same rigamarole to join a new Slack team.