Double-walled tents separate the sleeping cabin (inner tent) from the weatherproof shell (flysheet). The primary reason for this type of construction is to deal with the water vapour produced in the inner tent (condensation from the occupants breathing, damp clothing, etc.), which needs to be dispersed effectively so that the sleeping cabin can remain dry.
As the fabric of the inner tent is permeable to moisture vapour, the damp air can escape and condense on the cold outer surface of the flysheet.
This is even more effective if the dampness can be expelled by a flow of air between the inner and outer tents. In order to achieve this, Jack Wolfskin tents are equipped with a sophisticated ventilation system. This usually consists of a combination of air vents set into the walls of the tent, a slightly raised front edge to the flysheet and a tent entrance which can be partly opened whilst still protecting you from wind and weather.
The vents are positioned at different heights and on opposite sides of the tent to create a chimney effect and direct the flow of air.
Single-walled tents are only really suitable for very specialised use in situations where minimising weight is the overriding concern (high-altitude camps, alpine climbing, etc.) and condensation problems are of minor significance.