Why are there pictures in childrens story books?

They are intended to appeal to an illiterate or reading-readiness audience and generally the illustrations ( Carry) the tale- as one moves up the age ladder the illustrations decline. there was always a thin working line between comic strips and such things as say (Uncle Wiggily) which were a sneeze away and could easily go back and forth from ( Highly illustrated children"s book) to comic book- and did.