I went to tell my fortune, and the fortune teller said I was good, so I took my diary and wrote down my birthday. I said, why should I remember? I said I was a talent. Why a fortune teller? ...

Why do fortune tellers remember birthdays? There are several possibilities:

First, as you said above, fool people into spending some money with you;

Second, this person may really study the eight-character numerology, because your eight-character is different from what you see every day, or as the fortune teller said, or you are not sure, write down your eight-character for future reference;

The third is to accumulate numerology data and collect eight-character information.