Like others have said, this isn't really about being useful, so much as interesting or revealing. It answers the question "can we be certain that a Python program with type hints can actually be type-checked?" by saying "sorry, no: it's possible that the type checking will never finish".

The paper does that by hooking into some basic results of theoretical computer science: Turing Machines (TMs) can model any program, there is no program that can look at another program and say that it finishes execution (the halting problem is undecidable), and, oops, you can compile TMs into Python type hints, therefore type checking Python is undecidable.