Because its useful to have these data from a scientific perspective. Say for example you have leukemia later in life. A researcher wants to study what it is about your leukemia that makes it different from the rest of your blood. You can't solve that problem by drawing blood from the patient today, all the blood will be contaminated with leukemia. You could try and get DNA from the fingernails, but these are liable to be of poor quality and damaged by environmental factors that fingernails tend to be exposed to. Likewise for most other blood free sources of DNA you could take from a living person.

Now, if you had a blood sample from before the patient had leukemia, that would be invaluable data. You can yield high fidelity genetic material without much blood sample. Because of this, most states have laws saying this newborn screening material is available for various research purposes. The parent or the individual may also request their newborn sample, should new screening methods come out in the future that may not be on that panel when you were born for example.

https://www.cdph.ca.gov/Programs/CFH/DGDS/CDPH%20Document%20...