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.

You go to the doctor to get a physical checkup to see if anything's wrong, then get it treated.

This is exactly the same thing, but done for newborns who are much more fragile and vulnerable to make sure they get care while already in the hospital, especially for any potentially severe or immediate issues.

If you have technology to save a child’s life using simple modern medicine why would you not?