Is vocational training and an apprenticeship necessary to be a master carpenter? No, it is not: but it makes it far easier to become one.
And that's what a PhD is: it's a vocational degree coupled with an apprenticeship. It's not like a BS or MS: it's ultimately not about courses or grades. Rather, you are attaching yourself, like a barnacle, to a researcher -- the professor -- so that you can learn how to be a researcher yourself. Eventually you step out from under her wing and do some research on your own, and that forms your thesis work.
You could learn to do research in a specialized field without going through this process -- and I know several who have -- but you'll probably discover along the way that science or engineering research is an apprenticeship field for a very good reason.
And that's what a PhD is: it's a vocational degree coupled with an apprenticeship. It's not like a BS or MS: it's ultimately not about courses or grades. Rather, you are attaching yourself, like a barnacle, to a researcher -- the professor -- so that you can learn how to be a researcher yourself. Eventually you step out from under her wing and do some research on your own, and that forms your thesis work.
You could learn to do research in a specialized field without going through this process -- and I know several who have -- but you'll probably discover along the way that science or engineering research is an apprenticeship field for a very good reason.