Think of it as an extraction process. If you wanted to destroy the bones you could start with bone meal but that is NOT the point.
It can take 2-4 hrs to propely roast the bones which essentially produces crack for the water to penetrate. All in all this is the most important part of the process.
After that we look at two more factors, first one is the amount of pure water and the second is some heat to control bacteria and make the water softer.
A pressure cooker breaks both of those factors..
At the end you want broth full of water soluble minerals not mud.