I have 25 years of coding experience, built two successful businesses, and I have no idea what this thing is supposed to do.
Probably answering my own question, but the main reason I can think of would be if your app doesn't have some kind of business admin panel capability.
I suppose my bias is also that in my sphere of web dev, we build these business panels pretty frequently, so feature flags and a UI for toggling them is something that makes sense to do within the app rather than add a third party service (and associated cost + potential latency) for it.
However, there are settings that apply to all of them in our library code. For those, we use feature flags, and they are loaded from a network service, environment variables, code and config files. The overriding logic is complicated for legacy reasons and prone to bugs, so we are moving to a centralized feature flag system.