PM at Microsoft here. Live Share not only allows you to collaborate in real time in a lower latency snappier environment but also enables you to seamlessly transition from watching to participating, editing, stepping, and inspecting. The difference is you can collaborate but still operate independently which is not possible with something like a screen share. Screen sharing is great when you want to show your entire desktop or show visuals. Live Share lets you add another tool into your tool chest to better enable concurrent collaboration.