It is all about the area you have, you do not want to waste any square cm in passages, every space should be dedicated for your cargo, this is Narrow Aisle and high bay racking which combine most of the advantages of Wide Aisle Pallet Racking, but with better floor utilization by reducing the aisle width or increasing the height.

Pallet Racking Systems are designed to meet the new generation of warehouses up to 15m high, designed around the operating tolerances of VNA equipment with aisles marginally wider than the load. Such structures must have a high level of rigidity and accurate manufacturing tolerances.
It comes with a lot of advantages:

Good Stock Rotation and order picking; 100% selectivity with fast Picking rate.
Average Floor area utilization with 50% net pallet area usage of overall warehouse area.
By increased operational height, can increase warehouse capacity by an additional 40% compared to other systems.
Limited pallet redundancy as the system can run at 95+% capacity.
RF controls on the MH equipment will ensure maximum pallet cycles per hours.
Commonly used with Pick Up and Dispatch [P&D] stations at end of each aisle.
Mechanical Handling Equipment:
1. Specialist VNA machine – requires super flat floor surface
2. Operates within guide rails, standard or low profile, or using a wire induction guidance system which operates to a maximum height of 15m