From Tom Warren, writing for The Verge:

Microsoft automatically starts a companion system distro when you attempt to run a Linux GUI app, and it contains a Wayland, X server, pulse audio server, and everything else needed to make this work inside Windows. Once you terminate an app and WSL, then this special distro ends, too. All of these components combine to make it super easy to run Linux GUI apps alongside regular Windows apps.

Web developers love WSL, and I have no doubt that they are going to love this too, but can you imagine looking at Windows and thinking to yourself “y’know what this sucker really needs is another competing UI”?