Drawing and swinging a bow to shoot to both sides without jostling the man next to you takes some space, which there usually isn't on the inside. Arrows hitting the stone sides of the embrasure will usually shatter and loose its impetus.
Allows a wider field of view without exposing yourself.

The picture of the gun position above:
The stepped space above to allow better elevation of the gun and minimal damage to the concrete above from its muzzle blast.
Explosions on the outer face won't add too much concrete dropping onto the gun and traversing capability. Absorbs the energy from direct hits quite well.
Uses less concrete for similar strength.
Looks impressive to scare you off from attacking.
Provides material for discussion many years later.