i can explain it. (ok, i can't explain why someone would create a react monster instead of using a better suited frontend framework, but i can explain why i prefer to separate backend and frontend)
ever since i started using frontend frameworks for websites, the backend has become trivially simple. so simple that i have been able to reuse the same backend for all my websites built since. most websites do not need more than CRUD on the backend. and even for those that do need more backend logic, separating front and backend makes both parts easier to develop. they don't get entangled, and i can make drastic changes in either part without breaking the other.
frontend frameworks allow me to think of the frontend as an independent client application that runs in the browser. instead of having a complex application that displays here but runs over there, i have two simple applications, one that runs on the server and one that runs in the browser.
using a frontend framework makes development feel like writing a desktop application. i don't have to consider network latency, except to load and update data. but that can happen in the background. it doesn't affect the UI. it makes me think about loading data from the server efficiently. it allows me to preload data in the background. it allows me to cache it so that when the user switches between different views, the data doesn't have to be reloaded, yet it can still be updated.
you can do this with a server framework only too. but getting that working smoothly across multiple page loads is a lot more complex than having all views that share data in a single page application.