Slack is for most employees a way to socialize, to get connected, to be not alone because employees are actual lonesome creatures looking for community, looking for something to belong to. Heck, companies are for employees the same. They want to to find friends, to get laid, to network because they can't outside of their free-lunch-corp. If they had to work in the basement in a shitty 3-people-firm, alone, they would have run away the first day.
I haven't been employed for a long, long time, so my view on employees is quite negative and opinionated: employees except the sales ones are in terms of social interactions, networking, finding friends compared to non-employees way underdeveloped (to use a polite term). Don't confuse hanging around with peers in a company being social. Most wouldn't be able to find close peers outside of their company and comfort zone.
Hence, they need Slack so urgently, so they can chat, plan boring get-togethers and like each others messages with crappy emojis.