> I never really understood why Javascript developers wanted a library to help them manage state
The reason is rooted in how browsers work.
In native software you can "just" render your data structures directly to the screen. In the browser, barring webgl, you can't render your data structures directly to the screen, instead, you're forced to interact with an imperative browser API to individually manage bespoke UI widgets that store their own state internally and can't understand nor access any of the data structures defined for your business logic. The only option left is to manually update the internal state of the browser UI widgets whenever you update the internal state of your application. Manually keeping the two states in snyc is a real pain, highly bug prone, and a maintenance nightmare. The front-end browser frameworks came along as a way to "auto-sync" the business logic state with the browser ui state.
That's the same way you work with literally every other UI framework (except the immediate-mode GUIs popular in gamedev), complete with the built-in event loop that's hidden from you. Web isn't special here, except that no other framework I've seen allows you to modify the UI by programmatically editing the serialized text description of it.
The reason is rooted in how browsers work.
In native software you can "just" render your data structures directly to the screen. In the browser, barring webgl, you can't render your data structures directly to the screen, instead, you're forced to interact with an imperative browser API to individually manage bespoke UI widgets that store their own state internally and can't understand nor access any of the data structures defined for your business logic. The only option left is to manually update the internal state of the browser UI widgets whenever you update the internal state of your application. Manually keeping the two states in snyc is a real pain, highly bug prone, and a maintenance nightmare. The front-end browser frameworks came along as a way to "auto-sync" the business logic state with the browser ui state.