IMO the main reason Slack became successful is the massive number of pre-built integrations that were (and still are) available even on the free tier. It meant

IMO the main reason Slack became successful is the massive number of pre-built integrations that were (and still are) available even on the free tier. It meant that you could use it as a message/log/notification aggregator for the various pieces of software you were already using. Furthermore, those messages/logs/notifications immediately became searchable so you could for example do a search for <customer name> and have ALL the stuff related to that customer show up in the results (with links to the source applications).