I chose arbitrary fields that I suspected most readers of HN wouldn't be in, be familiar with, or necessarily esteem just to make the case more obvious. For example, a Ph.D in dance.
There are lots and lots of PhDs in all kinds of things. You can get a Phd in videogames, food hygiene, librarianship, gender studies, ancient greek, theology.
I am sure they are (mostly) worthwhile, and I'm not knocking any of them.
I don't see how they would, by default, be seen to benefit humanity. Some small number might. A very small number. Possibly an extremely small number.
According to one random website there are over 70,000 new PhDs every year.
That's a lot of assumed benefit to humanity. Or.. is it?