Since ancient times, knights wore metal and leather armor to fight wars and protect their lives. Over time this evolved into full body armor that consisted of heavy plate armor designed to withstand impact from firearms. Over time this evolved into heavy duty protection such as the modern GEROW, MLE and ASTM plate armor types that we have today. Metal plate armor was developed for officers to wear in a two piece suit to protect their torso from firearms fire. Over time the armor was refined to make a full body protective armor that could withstand impact and was flame resistant. Metal plates that are reinforced by steel have a much better penetrative ability than non-reinforced steel.
Today it is very expensive and impractical to replace the hard armor plates of rifles and pistols with lead or copper because of the high cost of zinc. Modern metals such as the stainless steels and the alloy steels are much more feasible to use as hard armor plates because they are much stronger and can withstand a much higher velocity caliber of ammunition. The main advantage of hard armor vests is that they protect the user against handgun projectiles at close range without decreasing maneuverability or accuracy of the user. A two-piece suit or plate carrier provides more protection to the user while being lighter and less cumbersome to carry.
A ballistic testing v50 test that was conducted on the NIJ standard showed that a vest with a one inch plate carrier weighed about three pounds unloaded. This type of soft armor allows the user to rapidly maneuver for tactical or combat applications and still maintain a good level of ballistic performance. NIJ tests also showed that the hardness of the armor made little difference to the dispersal of impact upon impact. It also does not affect the probability of injury to the user. This means that soft armor vests provide a similar level of protection to users at a much lower cost than steel armor.