It's used by companies to not only communicate, but also used by management level employees to pry on conversations/gossip. Personally know of a well known company where a group of managers used Slack for snooping on those they manage(d), and subsequently fired those they found criticizing them/higher-ups.

Not saying that this is surely what makes it valuable, but could see a management team deciding to use it as a means of improving productivity, where they could take logs and tie them into a system for gauging sentiment. Not saying it's right at all, but could see the value others could see in that