The Box and Cube is a material to offer to your baby once they have mastered the egg and cup.
Montessori materials are offered to children in sequential order going from the easiest to the most difficult. They are always building on previous skilled learned.
The square dimensions mean the baby has an additional challenge and he / she has to line up the corners of the cube and the box in order to succeed in fitting the objects together. More precise hand movements are necessary and so the child refines his / her hand movements even further.
This material give the baby further opportunity to practice using both hands together.