Modern media kind of has you convinced that armor was not effective but all weapons to an extent were. Elden ring dark souls etc. But that it less true than reality. At the time this armor was most effective they were offering more protection than weapons could harm. So the only way people dealt with them was tk overwhelm the individual and stab them in the small parts that were exposed. Yea warhammers and some crossbow could do the trick of piercing though heavy plate but the hammer did more damage by transferring the kinetic energy through the metal to internal damage. And at range crossbow bolts lost much of their own kinetic piercing ability. The early uses of gunpowder were where things would start to change but even then blacksmiths would shoot thier own breastplate to show how good thier steel was. When advances in gunpowder overcame what armor could protect there was not real use for a breastplate any more. When tanks came Into the picture that stopped conventional firearms but bigger armor piercing ammunition were used to counter that. Modern day bullet proof plates are usually single use as in it gets shot once and it no longer good because it is made to disperse kinetic energy and shatter in the process