Putting a bunch of Easter eggs in a movie really shows an appreciation for the fans, as well as an overt acknowledgement of their passion.
They say that fundamentally, the director gets that there are people who will watch the film closely and obsessively, and so they pack it with hidden clues and gags that reward repeat viewings. This requires an audience that truly cares, and that is especially true with the horror community.
In addition to hidden bits of foreshadowing, these films will often throw in at least one moment that's a love letter to the horror community and proof that the filmmaker is as big of a nerd as you are, perhaps naming a character after a classic horror director or paying homage to an obscure slasher flick.
After all, this genre's fan base is a tight-knit group all speaking the same language, and so the director likes to prove that they're speaking that language, too.
The hidden references usually aren't shoved in your face, but the more concealed they are, the most satisfying it is when you finally notice...