Tech support, guidance on opening bug reports, etc. There are some projects where there's not an open forum to post questions, but opening an issue or filing a bug report will immediately get your head bitten off by an angry dev unless you've read X number of docs, combed through closed issues to make sure it hasn't been asked previously, reproduced the issue, provided a GDB stacktrace of the issue occurring, etc.

IRC support channels can provide some immediate front-line support and allow you to ask basic questions without being told to go die in a fire.