I recently wanted to do a small project in React and found this site: https://reactjs.org/docs/add-react-to-a-website.html. It was a painless, plug-and-play solution that got me coding within 3 minutes. I didn't need any data from external APIs. Obviously Gatsby would have been overkill here -- but that left me wondering why anyone would use Gatsby at all, since it's billed as "the static React site generator", and I had done that easily without it.

This tutorial is appropriately concise and answered my question perfectly: Gatsby's main use case seems to be when you want to repeatedly generate a static site dependent on some data source changing (e.g. blogs, online stores, etc.)