FAQ

Because IT knew that it could continue to use Henry to create more fear/pain in the town for IT to feed off of.

'IT' appears to be a multi-layered creature that exists on more than one dimension, living in part in Derry, Maine, in part in the "Multiverse" that contains our universe, as well as in Its true form. Its true form appears to be a kind of mass of orange light (the "Deadlights") with an endless hunger that drives insane anyone fully exposed to It. In Derry, it has the ability to shape-shift depending on the fears of the person/child it is hunting (and it hunts children in order to devour both their flesh and their emotions of fear), taking forms like a Werewolf and others. It's "true form" in this universe (which is not actually its true form, but merely the closest representation possible in this universe) is that of a giant spider. It is intelligent and sentient (the book actually has several parts from Its perspective) and uses this intelligence to lure in potential prey.

According to the book, it is ITs heart.