Because material designations give you the details of the metal designated; such as yield strength and tensile strength.It is also important to make sure you have the entire designation ie: A516 is not enough information. You must also put the grade down; for instance A516-55 says the Tensile is 55-75,000 an the minimum yield is 30,000 while A516-70 says the tensile is 70-90,000 and the minimum yield is 38,000.