In exact that same scenario, that you would cast to a subclass in another language, it's about language support for what for example the kernel does with contai

In exact that same scenario, that you would cast to a subclass in another language, it's about language support for what for example the kernel does with container_of. Of course casting to a subclass isn't guaranteed to succeed always, but for example when you have actually declared it as the subclass elsewhere it's fine without checking for isinstance.