Copper/Tin Armor is a good starting armor in the game. I always skip wooden armor and make Copper/Tin Armor until I'm able to craft better armor like Tungsten/Silver or Platinum/Gold Armor.
Every item in Terraria has its valuable use, as I learn from playing Expert and Master Mode. Though Copper/Tin Armor doesn't give a lot of defense it's still a useful armor very early game compared to any wooden armor variant.
Saying Copper Armor is useless is like saying you can't beat the game with a Lawnmower (Which of course Sorbet Cafe has actually managed to do in one of his YouTube videos).