You're missing the whole picture. Branch.io was actually quite popular among a number of big app developers. A big part of what they did was deep linking into your app, and I may have the timing on this wrong but I think it was before iOS or Google had any deep linking functionality released. For example, you see this all the time on normal Android searches now, where if I have an app already installed and I search for, say, "Miami real estate", since I have the Realtor app already installed, Google's results come back and say "App installed", and when I click on the result it opens the Realtor app to Miami.

A big part of Branch's product, IIRC, was their "deep linking through install" functionality. That is, if I search for Miami vacation rentals, and I don't have the app installed, it would just open, say, the realtor.com website for Miami. But there would be a prominent "install app" suggestion, so I could then download the app from the App or Playstore. Importantly (and, again, I may be getting the timing wrong), at that time there was no way to "pass through" a desired landing page after the app was installed. Branch's links allowed that information to be passed through, so when I opened the newly installed Realtor.com it would open right to Miami. I can't remember all the tech details of how they did this but I believe it was some form of device fingerprinting.