For show or safety?

In the 17th century, soldiers usually wore some form of armour, most notably breastplates, to protect themselves against firearms. However, firearms became more and more powerful in time and the thickness of armour had to be increased to be able to offer enough protection. After armour had lost its protective quality and soldiers abandoned wearing armour, leaders still wore armour to indicate their status, so the armour was for show. But how much protection would a breastplate really offer the wearer?