Allow joining a call from more devices (ie: mobile app _and_ desktop app)
Slack allows joining a call from multiple devices, which is very useful for remote teams, if you need to be on the call but you're not at your desk, when you get to your desk later.
Or if you're in a call on mobile, but you need to share the desktop screen.
Steve Marshall commented
We had this facility in Webex and it was extremely useful. Sometimes I would join audio on a mobile device and just use the desktop for viewing a screenshare, or was out and about an needed to join the desktop version later.