In the CPC class of invention patents, that I am searching in, a lot of drawings show aspects of a invention, that aren't explicitly described in the text. Does that aspect of the invention still count as prior art? Would that be an example of common knowledge?
You have two good answers. I'll focus on your side question about "common knowledge". I'm not sure what you mean but only things in patents that are in claims are protected. All other information that may be new is "dedicated to the public". The catch is that there may be continuing applications that are part of the same family. Those applications can claim material that was unclaimed in other members of the patent family.
Until there are no pending applications the unclaimed material could be claimed. For your main question, the usual short-hand answer is "its good for what it teaches".