I've been watching interviews with King on "IT" and he mentions this. Sure, his stuff is graphic and gory at times, but that's not good horror. You have to build up empathy and sympathy for the characters, get to know them as people, before you tease them with the loss and terror to come.
I re-read the opening chapters of "IT" the other day and it's brilliant as a sample of this. It could literally be a short story of Georgie, since he's the "protagonist" of those chapters, showing his bond with his brother and how much he values him while wishing he could grow up out of his childish fears. Then, LONG before you get to the storm drain scene, King just outright says Georgie was heading towards his "strange death" by doing so and you know immediately this kid's going to die and now you're wondering how and, perhaps, hoping King was misleading you or misspoke and that a twist is coming. Only it doesn't. We know the kid never comes back alive when he leaves the house and that tension in finding out the details is horrifying since the previous paragraphs were all about how how close he was to his brother Bill.
When you care for a character, you want them to survive, and when a character you care about dies, you then know that any one of them, no matter who, could be next.