Every shot has a purpose for being in the edit.
Your not just animating an alien biting the head off a victim. You're showing the audience that these aliens are too fast to hit, too strong to fend off, and so aggressive that they'll charge in and take a couple rounds without flinching.
When it comes to characters, the goal is to show that the character is thinking. Actions are just the physical manifestation of thoughts. An Animator needs to reverse engineer, and reveal thoughts by showing actions.
Ideally, an animator will get an audience to feel empathy for the character. The only way to do that is to create emotion, by involving the audience in the character's thought process.