SF/Fantasy Movie Cliche #5, 674: "I am a powerful, vicious, skilful predator, but I will give you a chance to flee/attack while I roar and look scary for the cameras."
I have seen this moment in so many films I could not possibly count them. The hero is in peril, menaced by some powerful threat. Yet he does not know it. The enemy is creeping up on him unawares. Skilfully, silently. Soon, it will be in striking distance, and then it will be a simple matter to dispatch him effortlessly, quietly, and efficiently, like any true predator would.
But instead, the creature waits...and waits...for no apparent reason...until the hero finally turns around and notices it...whereupon, instead of administering the coup de grace and tearing out his throat, the creature just goes "RAAARRRRRRR!!!" and waves its claws around for a few seconds while the camera zooms in on its face - and the hero runs away or attacks, whereupon an exciting chase or fight scene ensues.
Sometimes it is even more patently absurd. This is probably the most egregious example from recent years. Watch what the red thing does when it has Kirk at its mercy, and tell me it isn't just plain stupid.
Solitary predators are efficient, cold, methodical, and stealthy, and even then they do not tend to catch prey very often - not even half as often as they try - because catching prey is really hard. If predators spent 10 seconds roaring at every single prey creature they wanted to catch, the prey would always run away and the predators would all starve.
Among the many, many lazy things that Hollywood directors and screenwriters do, this ranks pretty highly. In that one moment - the close up on the creature's face, the ferocious roar, the CGI saliva - a huge mass of cheap shorthand is communicated to the audience. Instead of building genuine tension and excitement through skilful direction we get a mere sledgehammer: THIS IS A SCARY MONSTER!
It it also often used as a naff plot device to get a character from A to B - in Star Trek, for instance, running away from the ice monster is how Kirk ends up bumping into Old Spock. It's because, you see, the director doesn't credit his audience with having an attention span longer than a gnat's, or himself with enough talent to maintain our interest without something loud happening on screen; it isn't enough to have Kirk just meet Spock. There has to be STUFF HAPPENING! at all times.
Predators don't roar at you. They aren't out to scare you. They're out to kill you. That's what they do.