Marsupials carry their young in pouches because the pouch is a life support system for babies that are developmentally equivalent to placental mammal features. The pouch is the way the infant continues their gestation on the outside of the mother's body. Inside the pouch, the offspring attaches itself to one of the mother's nipples and stays connected for the length of their development to the juvenile stage.
The young are born very underdeveloped, and the pouch is essential to the development of the young. Also, many predators can easily prey on a young joey, (baby kangaroo) and while adult kangaroos can defend themselves, their babies cannot. It is not only for development, but it is also for protection.
Marsupials is the correct spelling. The baby is born very tiny - just a few centimetres long - and immediately after birth it climbs up the mother's fur to sit in the pouch. Kangaroos keep their young (Joeys) in their pouch for several months, usually eight, until the joey is strong enough in the legs to jump on its own. Kangaroos move by jumping, not walking or running, and for this they need particularly strong legs and tails.
The joey would be very vulnerable if it came out of the pouch before its legs and tail had developed enough strength for this jumping. Otherwise, it would be a sitting target for any predators. Kangaroos are careful mothers and only have one baby at a time.