there's nothing to sync, the state is only in the backend. if you tell the user that the TODO is checked and you're only keeping track of it in the frontend and you don't sync it to the server and it's lost in the meantime, your UI lied to the user when they checked it and it showed as checked. With state on the backend, the user doesn't see that their data is saved until, you know, it actually is. And if all the state is rendered from the backend it can't get out of sync with the display

I hate it when a UI tells me I did an action when really there's an asynchronous background task happening that may fail