There are several possible reasons: functional, cultural, and economical. Steve Taylor covers the functional pretty well. read more
I am not an expert on all of the various types of shields; however, essentially, when you were carrying a shield you were balancing two factors - functionality, and weight/size. read more
Some types were used by infantry and ground troops. The History of the Medieval Shield. Shields, as in any type of armor or weapon, developed and changed over the centuries. They also changed in shape and size. They were typically not larger than three feet in diameter and made of wooden planks that were laid next to each other. read more