This is a broad question since you can obtain a Ph.D in so many fields.
One thing obtaining a Ph.D. can get you is practical experience. For anyone who has done it, reading a paper and saying "okay, I understand the concept" is a much different thing than actually doing the experiments that produce the paper. This is important because while you are doing experiments, you will often come across situations or problems that need to be solved. It forces you to think in ways you wouldn't if you only read the paper (e.g. "final product")
ANother thing a Ph.D. can provide is resources. Often times, Universities have equipment and resources you cannot obtain on your own. For instance, my thesis looked at the mechanisms of action of abusive drugs. Were I to try doing something like this on my own, I'd be breaking the law.
Now that I'm in my post-doc, I am doing human research. The resources involved are tremendous (nurses, physicians, equipemtn etc.). It would be impossible to do something like that privately.