SubFuze
A concrete example probably does a better job of explaining how this is used: https://medium.com/@dschnr/better-reusable-react-components-...
Referring to the Uber Freight RadioGroup and Tag Edit overrides from that article (not my article to be clear), those are cases where the vast majority of functionality from the original components satisfied the usage requirements, but a small tweak was needed to get the desired outcome. The override pattern is simply a way of building flexibility into the components and acknowledging that the users of the components may want to change them in ways that were not obvious to the component author. Those two examples may be fairly straightforward, but at what point do you stop with exposing the ability to change things? In our prior component library we saw a number of cases where teams wanted the ability to customize things that were more subtle, ie- changing the html element type on a subcomponent for a very specific purpose. This pattern came from observing the usage of our prior internal component library and talking with many of the internal consumers. It's probably not be a fit for everyone, but it works well for us.
You can use the components as is, using props if you're ok with the out of the box style/functionality/etc, without ever touching overrides. The overrides provide a standardized interface to changing the component internals across the entire component library.
That approach may be overkill for simple components that are trivally composable (Card for example), but is incredibly powerful for a component like Datepicker where you may want to change styles or functionality of a deeply nested component without exposing a massive top level props interface.