Why cherubim on top of the ark of the covenant?

What is the deeper message behind the keruvim—are they simply decorative sculptures, purely for aesthetic purposes, or is there more to it than meets the eye?

http://mosaicmagazine.com/picks/2014/02/childs-play/?utm_source=Mosaic+Daily+Email&utm_campaign=b29acebf8b-2014_2_21&utm_medium=email&utm_term=0_0b0517b2ab-b29acebf8b-41159925