Even though Slack is a web application, most people run it in the desktop client. For users with two monitors, the Slack app is running in one of their displays for most of the day.
People are joining multiple Slack teams: they’ll have one for their running club, one for their local startup scene, one for their coworking office, etc. The client allows people to see all of their Slack teams in one interface.
This is why Slack is so sticky: they’ve combined support for multiple teams with a desktop client that sits apart from the web browser.