Right, but the reason it was kept in a hidden field was that it would then go to the server on form submission and the server would persist it across requests,

Right, but the reason it was kept in a hidden field was that it would then go to the server on form submission and the server would persist it across requests, if I recall correctly – haven't done ASP.NET in more than a decade. Either way, that was a form of hydration itself I suppose, the server would send this state value to the client and the client would hydrate its view to match the state.