what is the use of a centre pouch?

Answer 1

Unless you meant "center punch" (a pointed tool used to mark a spot for drilling), a centre pouch is an interior, fold out, storage space inside a backpack. It allows for convenient organized access to the objects carried there while still allowing for access to the outer pouch of the backpack. It sort of reminds be of the airman's "B4" bag which folded open in a similar fashion but was carried by hand instead of on the back.