You are right about the scarcity of widely used research-related webapps. There aren’t many (if any). But the reason to publish one’s work as a webapp is not because it’ll be super popular or even useful to tons of people. The reason is that it’s one of the (potentially) easiest ways to grant people from group #1 access to your work.
Again, the main crowd that would use the app is composed of researchers that want access to the functionality of your code. They don’t know how to code or even use a REPL. They just want to use the analysis/solver/processing you developed to solve their specific problem. They are totally fine with waiting a few seconds until the webapp is done processing their request. They are totally fine with clicking on a bunch of settings every time they want to use your app. “They” is probably less than 10 people.
But my main argument for such a webapp building tutorial is not the number of users such an app would have, it’s the number of builders. It’s quite common to have colleagues that don’t code but want to use your “program”.
So you see, the fact that the app would be data intensive, domain specific, and interesting to only a fraction of users doesn’t diminish it’s relevancy. The ability to build webapps is still enormous.