Why was the Revolver or six-shooter made?

Answer 1

There were probably more factors that contributed to it, but...
You could guess that carrying more than one, ready to fire, bullet in your gun would be advantages in a situation where you would need said firearm. A revolver has that on say a muzzle loaded pistol.
A revolver, like a Colt Peacemaker, is mechanically simpler than a semi-auto like the Colt 1911. Which would explain why they were invented and produced first.
Also, generally, a revolver will produce more power than a semi-auto or automatic pistol firing the same round.