Solid implementations of autosuggest inputs, token input fields, or fast scrolling tables can run into the thousands of lines. Having reusable versions of them would be very helpful for the React ecosystem.
I have a couple components I can extract from a running project and publish there without any changes (Google Maps, Google Autocomplete, Tagging input, ...) and it will just work.
Want to render a sparkline of some data? Sure, you could mess around with existing libs and see if you can get them to work nice with React, or you could just pull in something you can expect actually works, like react-sparkline.