Hi Regin and Welcome. There will probably be better answers than this but, because he fed on fear, he was able to peer into each child's subconscious and 'know' what would best scare them, thus eliciting an even more tasty ephemeral snack! That is why, if stared at by 3 different kids, they may see three different Pennywises', each according to his or her own personal fears. Mmmm...tasty!
Well Regin, according to the book, It feeds on people - ye, literally, and "fear spices the meat", thus, making human flesh tastier. Or at least this is what I make out of It's own thoughts when It thinks out loud for the reader towards the end of the second tome. It is also described that It is mainly after children, because their fears can be summarized in a horrible monster, adults' fears are much too complex for It to manifest.