Why Is Pointing An Important Milestone For Toddlers?

Mostly because not pointing is an important diagnostic for autism.  Autistic kids don't point at things before 18 months in most cases.  Some autistic children/people never point at all.

Pointing is considered a 'proto-declarative' action.  The child is either asking for something, or saying to someone else 'Look at that!' with this gesture.  Which means that the child understands that you have a different perspective on the world from himself.   You are a separate person with a separate mind.  An autistic toddler assumes that the adults around him knows what he knows or wants, so there's no reason to point at anything.    When an adult points at something, an autistic child looks at the pointing finger.  A non-autistic person will look in the direction the finger is pointing.

It also matters which finger the child points with.  Ideally it should be the index finger (either hand) most of the time.  Using other fingers in preference suggests that wiring in the brain might not be exactly right, which is effectively the problem that autism is, too.

It is essential to cognition