A philosophy Ph.D. is not required to do philosophy, but neither is a physics Ph.D. required to do physics. But, while not required, such degrees are considered practically necessary, in the sense that the likelihood of contributing significantly to either field without having one is exceedingly small. Partly of course this is because of a selection effect; only people dedicated to the field are likely to make useful contributions, and people who are dedicated to the field tend to obtain the relative degrees. But I wouldn’t discount the process of getting the degree as making a difference either. Here, however, physics and philosophy diverge. Working towards a degree in physics is to develop an understanding of method and current physical theory that is necessary to do any serious work in physics. The same cannot be said for a philosophy degree, because there is a great deal of debate as to whether philosophy even has a distinctive method, and more about what that method would be. Similarly there isn’t really a state of the art when it comes to philosophy; certain theories are more popular than others, but few treat them as definitive.
A common misconception is that working towards a philosophy degree forces you to read all the historically significant philosophers, and that such a foundation is required to do significant philosophy. This is flawed in two ways. The first is that a philosophy Ph.D. only requires a thorough understanding of the history of philosophy if you want it too. In my program I think you could get away with reading as few as two historical philosophers (and two modern ones, thus allowing you to take issues classes for the remainder of the requirements), if you tried really hard. The second is the idea that you must read the great old philosophers in order to do philosophy. That’s simply not true. What is required is a familiarity with the issues and the major theories relevant to them (as well as a handle on the recent literature). Often that knowledge does come through reading the great old philosophers, but I don’t see any reason that it necessarily would have to.
But you can’t avoid having to have a handle on the current state of philosophy to contribute significantly to philosophy, even if your focus is on the history of philosophy. And working towards a philosophy Ph.D. does force you to keep up with current philosophy, whether you want to or not. Trying to contribute to philosophy without being familiar with its current state would be like trying to create a new theory of gravitation without knowing general relativity, you could try, but you would be unlikely to be successful. Naturally there is a downside to this as well, by being familiar with current philosophy it becomes almost reflexive to adopt the standard approaches to philosophical problems (for example, it certainly makes life easier just to describe knowledge as justified true belief in passing rather then trying to spell out a nuanced and unique position when all you really want to do is outline a rough connection between knowledge and ethical beliefs). And it is possible that the standard approaches are significantly flawed, and that by developing our ideas either on the basis of them or in reaction to them we are prevented from developing the philosophical theories that would solve our problems. While that may be a high price it is a price worth paying, because without that familiarity there is the tendency to either wander away from the problem inadvertently (via radical re-understandings of all the key concepts) or to simply produce redundant philosophy without realizing it (I can’t even begin to count the number of times that I have discovered that what I thought was a unique idea had already been developed by someone else). And this cannot be avoided by focusing on philosophical problems that haven’t received any mainstream attention, because to know which problems haven’t received that attention would require knowledge of the current state of philosophy.
But the real benefit of the philosophy Ph.D. is not to the person who earns it, but to the community of professional philosophers as a whole. Philosophy suffers from the fact that there are fewer barriers to entry to doing philosophy, compared to other fields. To do math or physics takes significant skill and dedication, so the ability to produce a mathematical proof by itself shows that the person who produces it has some degree of credibility. In contrast any fool can produce a philosophy blog. Thus many people produce what they call philosophy, despite the fact that it doesn’t really have anything to contribute to philosophy as a whole. Philosophy Ph.D.s then indicate to other philosophers who is worth listening to and who can probably be ignored, by leaving only the people who have shown a significant commitment to philosophy. I know that sounds elitist (but hey, I’m one of the people being ignored), but it is simply a matter of necessity; there is too much to read and not enough time, thus professional philosophers reasonably try to maximize the expected return on their time by reading mostly other professionals.