The beginning of nourishment and sustenance for us as children is milk. It gives all that is needed to help at the start of life. But it would not do to stay with milk as our only source of food. As we grow older we need more than milk alone is able to provide. We need to be eating more substantial foods.
Foods that provide us with energy - so we can work, and vitamins - to help us combat sickness and to keep us healthy. It would not be long before we would find ourselves unable to survive if we relied solely upon milk.

Our spiritual life takes a similar route. We need milk as new Christians as it allows us to develop in our new Christian life.
As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
(1 Peter 2 v 2)

But just as children grow into adults we need to mature in our Christian walk. We need to learn to feed upon the ‘strong meat’ of the Word of God whereby we can grow in strength and find perfect and satisfying nourishment.

For every one that useth milk is unskilful in the word of righteousness: for he is a babe.
But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.
(Hebrews 5 v 13,14)