It keeps the RPi and dongle closer to the air temperature in the box.
The air in the box is also cooled better because it interfaces with all surfaces of the box.
For example you have a box temperature of 40C, then the RPi core might get to 65C without a fan.
With a fan, the RPi will only go to 50C, 10 above ambient.
As the inside box temperature is more or less fixed by the power dissipated and the surface area of the box and heat conduction to the outside, it stays more or less the same with a small fan blowing on the RPi/dongle.
Inside box temperature might also be reduced because the air move over the box surface improving heat transfer to the box material.