Disruptive turbulence arises if the primary mirror is not adjusted to the ambient temperature. This can be a problem especially with open Newtonian telescopes. Fans mounted below or to the side of the primary mirror ensure a faster adjustment by removing pockets of warm air from above the mirror.
The need for such ventilation increases with the mirror diameter. Of course, the primary mirror adapts to the ambient temperature even without a fan. However, the greater the mass of the mirror, the slower it reacts.
If the telescope is transported to the observation site in a warm car, or if the temperature changes significantly during the night, the thermal adaptation can be too slow with a large mirror.