Making the building structure airtight achieves several significantly important goals.
The insulation will perform as designed.
Insulation is designed to trap still air; it's the still air that provides the insulation. If air is moving through the insulation the performance of the insulation is severely compromised, by up to 80%. Making the building structure airtight ensures that the insulation performs to its maximum potential.
Minimising heat loss
An airtight building structure is more efficient because draughts are eliminated. In a well insulated building infiltration (draughts) is the biggest source of heat loss.