One hypothesis would be to prevent ad-blindness. If you visit the same layout day after day, after a while your mind learns to subconsciously detaches adverts f

One hypothesis would be to prevent ad-blindness. If you visit the same layout day after day, after a while your mind learns to subconsciously detaches adverts from the content, it will be like it doesn't even exist. New layout, the learning process has to start all over again.