In agile pair programming it is recommended to switch pairs every now and then so as to increase the bus factor of the team. That means, most people in the team should work on different parts of the system at different times so that everyone has an understanding of it.
Now it is impssible that everyone has an expert level of understanding of each part of the system. That's because people are urged to specialize in one area of expertise. If you are an expert database admin, what's the point of working on the user interface of the system when you switch pairs? You will not be able to do such a high quality job as someone who has extensive experience in UI design.
How can you increase the bus factor and make sure that you have specialization in your team?