In the 16th and 17th centuries, many people believed that dreams could foretell future events. This made finding out what a dream meant very important. Talking to a professional dream "interpreter" was one way of finding out. People also read books that they believed could help them interpret their dreams.
One of the most famous dream interpreters was John Dee, who worked for Queen Elizabeth. He was a brilliant mathematician and scholar, who was also interested in magic. Dee helped the queen chose her coronation date based on astrological readings, and he became a trusted royal adviser.
Some people think that he may have even inspired the character of Albus Dumbledore, the wise head of Hogwarts School, in the popular Harry Potter series.