There was an article about cerebral palsy and how the symptoms can start to show many months after birth. So parents might not be aware that their child has a problem with their motor development until much later. The article also talked about how early diagnosis and intervention is important.
The babinski sign is an excellent method to catch these problems early on. I think most doctors check for this and some other reflexes during check-ups. But parents spend so much more time with the child and can notice this also.