One of the most dramatic changes to Shockwave for the movies, other then his choice in pets and skeletal frame, is the presence of a mouth. Did I say mouth? I should have said: UNHOLY MANDIBLES OF NIGHTMARES (OH GOD WHY?!), which we shall now acronym as 'UMON' Of course, I had to wonder: Is there an ulterior purpose for this facial purpose, these UMON? Does Shockwave have some in-depth reason for his unique visage? Is it simply fear? Shockwave's grim maw, opening into that evil hiss, would definitely cause many people to go find a new pair of pants, and would be very useful in inspiring terror into the hearts of his foes. Or perhaps Shockwave is actually a vampire. Leaping onto unsuspecting Autobots, his teeth extend into a horrible proboscis that digs into the inner workings of his prey and drains them of Energon. Alternatively, he gnaws their faces off. Either way is scary 0_0 What do you think? Is the UMON simply a neat visual feature? Or it is proof that Shockwave can, and will, eat your very soul.....
It's so he can umon-nom-nom his enemies. Personally I'm not fond of a mouth on any Shockwave, but they gave him one eye and his 'ears' so I guess two out of three isn't so bad. I know this isn't G1 but a creepy mandible hissing thingy just doesn't give me any logical Shockwave vibes. I own the figure though and it's one of my favourites, so I must've liked something about the design.
He doesn't have a face so he has those to take other peoples faces in a desperate attempt to get one for himself so he can be accepted.
I'd say it's just a facial feature really, a bit like how the disks on the side of Prime's head rotate and move seemingly on their own accord.