The one heat source of a fan is the motor. As the blades turn, a region of low pressure develops in front of the motor and draws heat from the motor into the air stream that heats up the wind coming at you.
Covers all holes in front of the motor. The motor will still cool from the vents at the back. There is a slight backdraft as the blades turn that runs over the back of motor. This cools the motor and bring heat way from the front wind.
Without motor heat being drawn into the front stream, the fan is now cooler.
By a lot.
