For me, tools that I find useful are ones that make something no longer be my problem as an application dev. For JQuery, it was low-level browser implementation details. Sure, browsers were different, I just didn't need to care anymore.
For React, it's "Calculating what are the DOM changes that need to happen to ensure the UI reflects the state of the world". In practice, this means that I no longer need to be concerned with "What's the current state of the UI", and I'm not manually diffing things. To a large extent, it makes client-side UI code look a lot more like server-side UI code.
State is one of those things that's a PITA to reason about -- the less of it that I need to care about, the easier it is for me to make code that works. Reasoning and testing in one dimension (space) is a whole lot easier than reasoning in two (space and time).
For React, it's "Calculating what are the DOM changes that need to happen to ensure the UI reflects the state of the world". In practice, this means that I no longer need to be concerned with "What's the current state of the UI", and I'm not manually diffing things. To a large extent, it makes client-side UI code look a lot more like server-side UI code.
State is one of those things that's a PITA to reason about -- the less of it that I need to care about, the easier it is for me to make code that works. Reasoning and testing in one dimension (space) is a whole lot easier than reasoning in two (space and time).