The primary purpose of moon clips is to allow revolvers to function properly using rimless cartridges designed for use in auto-loaders. The moon clips hold the cartridges and give the ejector star something to push against.
I believe they were originally developed around the time of World War I. At the time 1911s were in short supply. So Smith & Wesson and Colt produced revolvers chambered in .45 ACP so sufficient numbers of sidearms using the then standard Army pistol cartridge could be available.
But since the .45 ACP used a rimless case, there would be nothing for the ejector star to bear against to eject spent cases. So the cartridges were fitted into clips. Originally they were called half-moon clips. Each held three cartridges so two were needed.
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper