Without the air deflectors the airflow is smooth (and hard) right close to the fuselage - meaning a para exiting via the door is hit hard by fast air as he is halfway out the door - not nice, and can cause injuries, and/or cause the para to start his jump tumbling/spinning rapidly.
The deflectors move the airstream out from the side, and provides a nice pocket of disrupted air outside the door, which is much easier on the body - allowing a more gradual insertion into the airstream, reducing injuries and improving the para's control of his own body at the start of the jump.