The Jaffa's "cobra" headgear rests high on the wearer's head, and the eyes of the wearer appear to be about midway up the cobra helmet's neck. At first I thought maybe they can look through an opening in the snake's mouth or something. But as you can see in this collage of screenshots showing the helmet opening, pic #3 clearly shows Teal'c's eyes obscured by the neck armor until the neck part slides down further to reveal more of the face.
How can anyone see anything out of the helmet when it is closed, let alone give any tactical advantage even if they do somehow have limited sight? I always felt any fear/intimidation of the headgear was vastly outweighed by how unpractical it is in battle.