Besides the React docs, the authors of a book called Full Stack React are currently doing a tutorial series called "30 Days of React" (), which has been excellent so far. Covers a lot of key topics. Also see my main comment above for my list of links to React tutorials and resources.
As for reasons to use React: I could talk about it for quite a while, but I'll stick with just one for now. Going from imperatively poking around at the DOM and toggling things to simple declaring what the UI output should be based on props this component was given and state this component controls is a huge improvement. It takes some time to switch your mind around to thinking in that style, but once you do, your code becomes much more predictable and straightforward.