Webpack Config via /r/reactjs

So I know what Webpack is and what it's used for. However, as a React nube, I have no clue why I should care, since I just run create-react-app, code my app, run npm build and deploy that shit. The things is in a few interviews I have been asked about "Webpack Config" and have no clue how I'm supposed to respond to that. Is it ever necessary to "config" webpack if using create-react-app? Or perhaps some folks don't use create-react-app?

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