[mentalhealth]

The way many metabolic conditions work is that essential products are not synthesized, or toxic products are not cleared. These can cause unrecoverable damage within hours or days of birth if not treated. So, tests are performed to identify known metabolic conditions to prevent newborns from suffering unrecoverable brain/otherwise damage. This is generally known as "newborn screening" if you are curious to learn more.

[hardwaresofton]

Not OP but maybe the question is more "why is the state doing this and keeping the samples" not "why is it useful"...