When most parents take their young infants and children for well exams, a developmental inventory is usually part of a complete evaluation. That inventory can be either observed in the exam room, or attested to by the parent. But most parents have no clue what things a child should or could be doing at any particular age.
Developmental inventories are good for parents to know. They are a starting point for Pediatricians who try to assimilate all of the exam information and be able to conclude that a child is normal developmentally or not.