Why were big holes made in them?
What difference would be there if you were to look from a straight flat wall or a bastion at a height? How would the soldiers find peeping from the holes in the bastions useful while attacking?
Big holes were made in them to watch and shoot at enemies from a distance, keeping the soldiers safe.
Bastions (Burj) built at a height provide the space for looking on distant places from several directions. With flat walls one could see in one particular direction only and not too far.
Soldiers could look in several directions through the holes in the bastions. They could also save themselves while attacking the enemies with the guns or cannons.