There are many possible reasons to wear sunglasses in a movie. Usually the sunglasses can make the character more outstanding and cooler. They can leave an impressive impression on audiences. But sometimes, if there are all kinds of sunglasses being used by the characters, it may be a way of advertising for this kind of sunglasses.
Oh, they are so cool. I have googled on the internet to see the pictures of matrix. There are many guys wear sunglasses and carry with a gun. I think the reason why these guys wear sunglasses is to cover their facial features. If other people know who they are when they are raiding or killing, they will be recognized and killed by others the next time.
Don't you think they are really very cool? Maybe the film maker wants to attract audience in this way. And maybe this can be a good selling point so that the film maker can make a profit from this. But I think this kind of movie can tell you how to choose a pair of sunglasses that are fit for your face shape. You just need to have a good look and then you may get some useful conclusions. Good luck!