I'm pretty sure shiny charm is for breeding, though probably I'm wrong.
This Key Item increases the likelihood that you will encounter Shiny Pokémon. When you have the item in your bag, then the chances of encountering Shiny Pokémon in the wild is increased to 300% of normal, making it a 1 in 2,730.6 chance. If, however, you're using the Masuda Method to breed, it increased it from 1 in 1,365.3 down to 1 in 1024. As of Pokémon X & Y, it only works on Wild Pokémon and increases the rate from 1 in 4096 to 1 in 1365.3.