A newborn kangaroo is tiny.
It is also far less developed than a newborn human, dog, or many other mammals.
The pouch gives that immature baby a protected place to continue growing.
Kangaroos are marsupials
Marsupials give birth after a relatively short pregnancy.
The newborn then completes much more of its development outside the uterus.
For kangaroos, that development happens largely inside the pouch.
The newborn crawls into the pouch
After birth, the tiny joey moves from the birth opening to the pouch.
It attaches to a teat.
There it receives milk and stays protected while its body continues developing.
Why is the pouch important?
The joey cannot function independently.
It needs warmth, nutrition, protection, and time.
The pouch provides all of those in one place.
It is effectively an external nursery attached to the mother.
That very early stage of development is one reason marsupial newborns look so different from the more developed human babies discussed in why babies have more bones than adults.
Does the joey stay inside until fully grown?
No.
As it develops, it begins leaving the pouch for short periods.
Eventually it spends more and more time outside.
A larger joey may still dive back in when frightened or tired.
Do all marsupials have the same kind of pouch?
No.
Pouch structure varies among marsupials.
Some open forward.
Some open backward.
Some species have only a fold of skin rather than a deep enclosed pouch.
How is this different from an egg?
Both strategies protect developing young, but the biology is different.
A marsupial joey is born alive and continues feeding from the mother.
The pouch supports a very early-born mammal rather than enclosing an embryo in a shell.
The pouch is basically an external nursery for a baby born astonishingly early in development.
And that’s the short explanation.
Related explanations: Why do animals have tails?, Why do turtles have shells?, and Why do babies have more bones than adults?