Sucking is a common, primitive and natural reflex, originated in the central nervous system, among all mammals including babies. Even before birth, many babies have even been observed sucking their fingers while still in utero. Sucking is one of 5 womb sensations known as "5S's"

Sucking makes infants feel more secure and helps them explore the world around them. On the other hand, sucking as a primitive reflex that has a survival value.  For instance, a baby will automatically suck at anything that touches the lips or the roof of their mouth.  This burst of sucking simulates the natural breastfeeding process by pressing the nipple between the tongue and palate to draw out the milk and moving the tongue to coax milk from the nipple to be swallowed by the baby. 

Sucking has the power to calm babies  as it lowers the heart rate, blood pressure and stress; it even eliminates crying after injections and blood tests.

When babies suck on a pacifier, toy or thumb, it's called non-nutritive sucking (because it yields no nutrition) whereas breastdeefing is called nutritive sucking. Non-nutritive sucking helps babies stay calm amid the chaos of the world around them. But as hunger builds, your baby will eventually spit the pacifier out, as if to complain, “Hey, bring some milk –I do not need rubber!”

Sucking usually disappears in 4-6 months after birth.