Working with colleagues spread across Massachusetts, New York, California, Oxfordshire, London, Britanny, Victoria, Lower Saxony (and that’s when they’re not travelling) means that face-to-face conversations aren’t universally available. Instant messaging is good because it’s the next-best thing to direct contact.
With geographical distance comes timezone distance. Before the invention of immediate communication this was a self-correcting problem (most people can’t shout over more than two timezones). Of course, when one simply has to type into a computer (caps-lock for shouting) one must be sensitive to the timezone of one’s correspondent. Easy advice to give, but also easy to forget. Slack, like any long-distance communication medium, has the effect of reducing people to avatars somewhat.