More generically, Metaclasses are primarily useful when using (abusing?) the Python type system to represent other type systems. The most common example of this

More generically, Metaclasses are primarily useful when using (abusing?) the Python type system to represent other type systems. The most common example of this is ORMs like SQLAlchemy, where the Python type system is used to represent tables in a relational database. Another example is the builtin "Typing" module where the type system is used to provide a language for representing itself. "Union" itself is a type, and metaclasses allow you to set "__getitem__" on the "Union" type such that Union[Int|Str] is valid and represents a new type. Not that I don't know if metaclasses are actually used to build this system internally, but you could use it for many of the features implemented by Typing. There are alternative ways to go about doing that because Python is so flexible, but IMO metaclasses (or __init_subclass__ as seen in this post) are the least "magic" of the magic ways to tackle problems like these.