Not every patient has a family history. Genetic diseases can exist in dominant or recessive forms. They can also be caused by acquired mutations.
There are now more and more rare disease treatments available, which some of the rare diseases are not incurable. For example, Spinraza for treating Spinal Muscular Atrophy (SMA), and Everolimus for treating Tuberous Sclerosis Complex. More pharmaceutical companies are willing to develop drugs for rare diseases. They choose to develop each drug based on the number of patients in patient registry. They consider the number of patients in the patient registry as one of the factors affecting which drugs to develop. A confirmatory diagnosis can help establish a patient registry. In a long run, it helps to make more drugs available.
Reference:
1. Eurordis – Rare Diseases Europe
2. Global Genes. RARE Disease Facts. https://globalgenes.org/rare-facts/