[nrp]

You can only dissipate so much heat passively conducted through the surface of the device before the temperature of either the internal components or the surface of the case exceeds what is safe. Getting around that often just means having some amount of forced air to carry heat away. In this case, it's a blower that pulls air in from vents, pulls it over various parts of the cavity (including some amount over that heatsink), and exhausts it out. Given how low the TDP of the CPU in a MacBook Air is, the thermal budget presumably balances.