Business is business. And sometimes, you don't have the time to create tons of components, style them and make them animate like it's no big deal. That stuff is tedious and time-consuming! So sometimes, a ready-made component library is the right tool for the job.
If you want to create an app that follows the Material Design pattern outlined by Google, there are actually several different implementations to choose from. The one I have the most experience with is Material UI - a pretty complete toolbox that let's you spit out clever-looking apps in no-time.
Two other implementations of the same design are MUICSS and react-toolbox. I haven't used them personally, but they both get good grades in social media and around the web. Consider them all, and use the API you like the best.
Bootstrap has been battle tested like few other libraries ever have , and of course there is a React component library implementing it. The react-bootstrap package contains pretty much all of Bootstrap, and even though it's still in a pre 1.0.0 release, it's being used by tons of projects online.
A third approach, and one of the best looking ones in my mind, is the React port of the Semantic UI library. It looks really good, has most of the components you'll ever need, and has some really nice documentation.
Tons of others!
If there is a design system that you really like, there is probably a React implementation of it somewhere. And although creating your own design system probably is what you'd like to do in the long run, using one of these to create a proof of concept or a simple one-off implementation is probably what you want.