My main answer here is latency. Let me explain.

For our app the back of the frontend provides the working data model.

So for our web app we have a next style server side renderer + serverless functions

For the mobile app we stick we a queue based architecture (the web back of the frontend resolves to the same underlying data structure.)

So imo different front ends have different use cases. But in general the back of the frontend isn’t a backend it’s the frontend server. So the round trip is the same for first paint, api requests etc. For the web app at least