Why were heraldic shields designed that way?

It is a visual record of who is what. In the Middle Ages, most of the population could not read. The heraldic shield is a paragraph in pictures. The symbols tell specific things about the family that uses it, including the family motto (which people couldn't read in the first place.) If you were wealthy enough to need a crest, you made some pretty strong statements so that you would be noticed. The shield was/is a defensive weapon and the crests were added to identify the commoners who fought under the family name. If you were important enough to have a shield, you were a valuable fighter. So identifying a fighter was also a way for the opposite side to concentrate on those experienced fighters who could do more damage.