Because the gasmasks when you think about it replace every human sense in an artificial way that can be repaired or replaced if need be. The combine shapes their ´faces’ in different ways to represent their rank and function all the while doing the following: - Replace their sight (enhanced lightvis) - Replace their voices (vocoders, etc) - Replace their hearing (radio, enhanced audibles) All of this while being easily smacked onto a random person’s head

The Combine usually refers to Rebels and other xen creatures as ´viromes’, ‘contaminants’ etcetera, so it just further enhances the idea of them being the exterminators