The left panel is a React/Redux application. The user's email address lives in six places: the Redux store, component local state, the form input's value, URL search params, localStorage, and a memoized derived value. When the user updates their email, the change propagates through dispatch, reducers, selectors, and side effects. Each location updates at a different time. Between updates, some locations hold the old value while others hold the new value. The application is in an inconsistent state until all six locations have been updated, and there is no single point where you can verify they agree.
The right panel is an HTMX application. The user submits the form. The server processes the change and returns new HTML. The browser swaps the HTML into the DOM. The email address lives in one place: the DOM, as rendered by the server. There is nothing to synchronize because there is only one copy.