We just use this protocol/schema to make sure that development URL links are tappable on your phone. Checking a URL domain works for lots of deep links, but not when you have IP addresses or dynamically generated ngrok URLs (like when loading an app from XDE).

You might ask, why do this when you can just scan or screenshot the QR code? Fun bit of tech debt: the current QR-scanning system was added to support the offline-only create-react-native-app setup, and the "send a link by SMS" flow predates that. If you've received an SMS with your development URL, it's pretty useful to have it tappable.

Related, this detail of the XDE <-> client app connection isn't really what people use for production, it's just for development and sharing with coworkers in most cases. To ship a "production" Expo app, users typically build an IPA or APK for uploading to the App Store and/or Play Store, where this URL is hidden from them.