Well Phd is an attainment of state where your mind should be boggling with intense curiosity - why some things happen and why other things do not? If you do no engage in research works after Phd (whatever your subject of interest be) then you have committed two sins: (1) you have consumed valuable resources (time/money and so forth) which someone else could have benefitted from, and (2) you did not understand why people want such a state of attainment possibily because you are the victim of status quo.
Phd is a process of learning and not just plain and incremental learning. The process of learning never ends and Phd gives you that ability to put the learning in context from which human societies can benefit (here societies and benefits both are subjective).