A map can show you all of an area, but to use it well you need either a compass or to have sight of some landmarks. The map won’t show every detail, but can show things that you can’t otherwise see. A compass helps to give you a direction to follow. Landmarks are useful to highlight your position.
This situation is, in some ways, similar to how someone can get a feel for what the viva is like.
The regulations for your university show a lot, but to really get a sense of what they mean in practice you need to find out about the general experiences of PhD candidates – or find out more specifically about vivas in your department. Regulations describe the big picture. Viva stories can help you to understand what to expect – and knowing about vivas in your department can help you see the hyperlocal practice where you are.
Regulations help you to map out the way things are supposed to be. Viva stories give you a direction to follow. Stories from your department give you the landmarks to know exactly where you are.