The first five minutes of a movie are everything. Whilst you don't want to set a precedent the film can't then live up to, the opening sequence is what is going to get your audiences to stick around, and in the case of horror movies: the bigger, the badder, the gorier - the better.
We want to know exactly what we're getting ourselves in for when it comes to the spooky central arc of the story, with filmmakers consistently delivering sample-sized scares to get a taste of the action coming up. More often than not, horror movie openings act as their own mini-narratives, delivering a separate precursory story to explain what our protagonists are coming up against without giving the game away entirely.
It takes time to build up intensity for monsters and to get us to care about the characters - so might as well bin some unknown off in the opening to sate the bloodlust for a while. We're a sick bunch.
For some truly bite-sized terror, there's nothing better than a eye-popping (sometimes quite literally) opening to get you going before you're knee deep into the good stuff: so you better bring your wellies along for the ride...