A special feature about Quatre is his goggles. Sometimes he wears it and sometimes he doesn't. If you have noticed during the series, everytime Quatre fights with the Maganac Corp, he wears the goggles. Why is this? Why not any other fight? Well... it all goes back to the time when Quatre was 13, when he actually met the Maganac Corp.
In the Maganac Corp, only Captain Rashid has googles, meaning that the goggles symbolizes leadership and the rank of captain in the Maganac Corp family. When a member of Maganac tossed Quatre the goggles under Rashid's command, Quatre has automatically taken the role of "captain". This is because when Quatre asked Auda to attack, he called him 'Mister Auda'. Auda replied that he shouldn't call him mister anymore because he's wearing the captain's goggles. I suppose Quatre wears his goggles everytime he is with the Maganac because the family acknowledges him to be the leader. Basically, it's the leader's mark. The same thing applies in the case of a captain's hat or a crown.
Personally, I think Quatre wears it to remember the day he was accepted into the Maganac family. Quatre usually wears it on his forehead and keeps it there. The purpose of goggles is actually to protect your eyes. We'd understand the use of it in a mobile suit. When the mobile suit is damaged and little mechanical parts fly out, it could hurt one's eyes. Maybe that is why people wear it. Quatre hardly wears it on his eyes though. =P Only time he did in the series was when he was in a sandstorm. He really needed it to see in that EP. The wind was blowing sand everywhere. X_x In general, I think leaders just wear it to show their status. Even Lieutenant Noin would wear goggles sometimes when she was commanding troops to attack.
The googles' stripe and rims are brown in colour. (I suppose brown is the universal colour for the Maganac. ^_^) Its rims take the shape of a hexagon and the lens are very clear for a clear view =). But during Endless Waltz, he wears goggles even when the Maganac Corp isn't fighting with him. I guess it's pretty stylish too. =P But for sure, everytime Quatre fights with the Maganac, you'll see him wearing goggles to show that he is the leader of the Maganac and he will protect them.