Short Answer: To get accurate information
There are many books and free trainings on how to read palms. These may or may not be accurate. Learning to read hands well takes a lot of knowledge and experience. Would you do surgery on yourself if you read about how to do it in a book? No, that would be ridiculous. The same concept applies here. A true and caring professional provides insights that aren’t in any books, plus they bring all their experience to help you in ways you otherwise might not have considered.