* Free. You can hop onto an existing tech-focused server like Freenode, or if you want your own control, throw it on your own server(s) for pennies. If you do the latter, you have full control over all logging.
* Logging. Every client can choose what they want to log, what format the logs are in, where the logs live, etc. No more archived messages. Some IRC clients are smart enough to pull from the log to "back-fill" the messages.
* Modern Clients. Like a said above, LimeChat is a really nice IRC client. It doesn't have all the features of Slack, but do you really need all the features of slack?
* Bot Surplus. There are hundreds of bots written for IRC, in nearly every conceivable language. This was the first "integration" anyone ever used for chatting, and can integrate with anything if you put the time in. You don't get it for free, but for a community focused on programming, that's not the worst thing.