Knowing how one specific system really behaves. Docs only get you part of the way. Recently I spent a lot of time running an old Microsoft API side by side with its replacement, and a good part of the differences aren't written down anywhere. AI reads docs well, but it can't know what's missing from them.
An entirely new job? Or do you mean a new job as a different type software engineer? I'm afraid that in these days of AI software development the major differences between two capable software engineers is their AI skills and, perhaps, their physical appearance so, brush your hair, dress nice, smile and dazzle them with your ability to steer coding agents.