[rocketflumes]

A PhD is not only about solving some specific problem. It is more about learning how to solve problems, and more importantly, choosing what problems to solve.

Having a PhD should mean that the person is capable of making consistent progress in ill-defined problems without much external support. Of course not all PhDs can do this, and you don't need a PhD to gain these skills, but they correlate strongly enough that it makes sense to just filter some jobs by whether or not someone has a PhD.