Earning your doctorate in philosophy (that's what 'PhD' stands for) in materials science and engineering is the culmination of becoming a professional researcher. What does this mean, exactly?
Well, for starters, professional researchers can do Research as their job. In materials science, this means demonstrating a breadth of fundamental knowledge that is needed to understand what materials are, how their structure, processing, and properties relate to their performance in engineering applications, and being able to use the scientific method to advance the current state of knowledge about the universe. The step of advancing knowledge is not limited to designing an experiment and performing the experiment: It necessarily includes explaining to other scientists and non-scientists what the results mean and why they are important. This last step of communicating scientific research in many ways distinguishes PhD-level researchers from those with MS-level training. The PhD has the broad knowledge needed to put their work in context that matters for the specific interests of their research field and to explain the importance of their work to the general public. So...