The time was when armor kept your character from getting hit in combat. D&D did it that way. Classic Traveller did it that way and we were happy.
Then The Fantasy Trip came out and lo! Armor absorbed damage from an attack. It no longer kept you from being hit. No more attack matrices and charts and such items dear to grognards everywhere. Many left and never looked back. They derided armor class/to hit people. This way made sense. Hitting a person was a matter of skill. Damage was a measure of the force that actually was transmitted to the victim. Even Traveller went with this. The Azhanti High Lightning and Striker games used a damage reduction system. Ditto 2300 AD when it came along. Realism!
Except it had its problems too. For example, a dagger really had no chance of piercing plate armor. Nevertheless many systems allowed a dagger to do a point or two on a good enough damage roll. Some didn't. But then that meant a sword and other weapons needed more damage dice or whatever. This could get really silly for modern weapons. In order to have low end bulletproof cloth stand up to pistols doing say 2d6 you needed armor that absorbed at least 10 or 11 damage. Now rifles can shred armor that can laugh at pistol rounds. That meant rifles had to do at least 4d6 to put a hurt on people. That's fine for dealing with armored folks but when you turned the weapons on unarmored folks you might all but disintegrate them (assuming 10 or 12 hit points. GURPS I'm looking at you.) A damage of 4d6 pretty much rolled 14 most of the time. If your PC's damage averaged 12 you were flat out of luck should you get hit. What, a rifle bullet never grazed someone or just took off an ear?
Then some bright designer got the idea of redefining hits in more detail. Roll a hit location to see where you hit. Some amor only protected the torso for example. Hit a guy in the arm and he was going bowling that session. Some armors had protection that varied by area. Lots of cruchy goodness there.
So in effect armor now could prevent all damage to certain areas. Hit a guy in the right spot and even a dagger could harm a man in plate armor.
Which sounds a lot like armor reducing your chance to hit once again. Also armor class tables were clunky but hit location was a real bitch in the opinion of many (or my opinion at least.) So which mechanic should you use given the choice, damage reduction or higher to hit numbers.
Depends. Armor class puts an emphasis on skill. A skilled person could shank a giant with toenail clippers. Damage is unrelated to penetration of armor and even unarmored people may survive a few hits. It's a good set up for cinematic play styles.
Damage reduction spawns gear heads. You're always looking for the best personal protection but then an npc comes with a bigger gun than yours. It can lead to an arms race. Skill doesn't matter as much because even if you can shoot the ace out of a deck of card you won't put a hole in that exo-armor suit with that body pistol.
Now if you'll excuse me I'm going to re-read Classic Traveller.