Slack tries to satisfy a number of requirements, all through a chat interface. 1:1 chatting, group chatting, team/project discussions, company updates, etc. I think Slack is as good or superior to Workplace just considering chat functionality, but as a tool for inter-company collaboration Workplace is much better.

Once your company reaches a sufficient size (maybe 300+ employees?), Slack channels just become so clunky and tedious for trying to keep up with projects and discussions. Using Slack keyword notifications helps cut through the noise, but I really think applying Facebook's ranking/Groups expertise to work-related content has been the best solution I've seen so far. You're no longer inundated with unread channels that may or may not have messages relevant to you, Workplace just surfaces the relevant content (and obviously there's chat/mentions for more pressing issues).