Originally Posted by Shao Kahn on the Lawn
The explanation is that Pennywise terrorizes first to "salt" the meat.
Terrorizing them is more like cooking the meat, according to the book's explanation. Pennywise can only kill you if you partially or fully believe in him. If you don't even realize he exists, he can't do anything to you. If you completely, 100% believe in him with no hesitation, then you are able to fight back against him.
He uses terror and intimidation to get people to believe in him kinda-sorta, but the outrageous nature of the scares is intended to make people uncertain that what they're seeing is real, so that they can't effectively fight back against them.
Also, spoilers from the book about Pennywise's backstory: It's actually an alien that crashed onto the planet before humanity even existed