What benefits are provided by a proper air seal?

Airtight and draught-proof implementation has numerous benefits for the users:

Structural damage is avoided: if air flows through permeable building elements from inside to outside, water vapour cools down and condenses within the structure, forming droplets inside the elements, which results in structural damage.
Effectiveness of thermal insulation: if air flows through the insulation layer, the insulating capacity of the structure is significantly reduced in practice.
Soundproofing: every leak makes soundproofing less effective. A proper air seal is therefore part of the soundproofing strategy.
Optimized ventilation: in the absence of a proper air seal, air is exchanged because of wind pressure or thermals, which are very much a function of the weather. Excessive air exchange then takes place exactly when it is not welcome: with strong winds and in very cold weather.
Thermal comfort: cold air coming in through leaks leads to draughts, pools of cold air resulting in cold feet, and an uncomfortable vertical layering of temperatures both in individual rooms and throughout the building.
Reduced thermal energy requirement: for the reasons given, making a building airtight leads to significant savings in energy and cost.