Coming from old classic SharePoint pages you might have existing script solutions you want to re-use on a modern page without having to repackage it as a new SharePoint Framework web part. This web part is similar to the classic Script Editor Web Part, and allows you do drop arbitrary script or html on a modern page.
Open this sample in the gallery (popup panel): Open in gallery
Source: https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/react-script-editor-onprem