Netlify provides a global cdn and administrative features targeting statically generated pages. S3 is more generic in its goals so it doesn't provide either.

Netlify provides a global cdn and administrative features targeting statically generated pages. S3 is more generic in its goals so it doesn't provide either. Heroku on the other hand can host server applications which Netlify cannot so it is really a different service. You could host an API on heroku and the react app on Netlify.