It's just a duct, to direct the air onto the cards instead of it going everywhere.

Airflow from that fan without the plastic duct would look like this:

http://www.youtube.com/watch?v=8m8fC809TK0&feature=youtu.be&t=47s

Compared to with the duct it would look more like this (but not exactly):

http://www.youtube.com/watch?v=8m8fC809TK0&feature=youtu.be&t=1m47s