Re: Why oh why?
People pay for it because they've created APIs that are easily usable for integrating other products without having to partner in a closed relationship. They weren't the first, but getting new start-ups or deploying into SMCs is easy. Cross platform as well.
Try integrating with IRC (yes, people could to a degree, but it was mostly file transfer based plugins), ICQ, MSN Messenger back in the day. They were built with consumer, not the pro in mind. WhatsApp/FB Messenger/SnapChat have taken over that market. The professional market was still open and Slack/HipChat came in to revive that market.
I've personally used both. HipChat caught up quickly with stability being a sticky point (especially in Windows where that version of it was pretty unstable for a while).
Pretty sure Slack will get cheaper over time. Running lots of architecture to support instant file uploads/sharing is difficult/expensive. Microsoft always comes in at last place and then dumps it after a few years when they haven't bothered to nurture it.