Sometimes it is required that a fan is able to operate at elevated temperatures continuously, or when there are particles or hazardous gases within the air stream that would damage a motor if it was unprotected. Bifurcated fans split the airflow around a motor tunnel which means that the motor is held outside of the airstream and therefore does not get exposed to what the air stream is carrying.
High-quality fan designed to withstand continuous hot temperatures and with a protected motor that does not get exposed to any particles within the airstream