Scanning QR codes is another new feature in the Pokémon series coming in Pokémon Sun and Pokémon Moon, and it helps you to complete your Alolan Pokédex. Scanning a code once a day reveals the location of this pokémon on the island you are on to let you catch it and keep it forever on your list.
After scanning 10 codes you get 100 points that you can exchange for an Island Scanner. This is an open door to rare pokémon, because when you do the Island Scan a Pokémon not from the Regional Dex may appear, but only for one hour. Serebii has a list with the Island Scan Pokémon, and among them is Chikorita, which is not very easy to get legally.
But to get all the QR codes is not an easy task as well. Players and Reddit readers are working together to create different lists with the complete Alola Region Dex QR codes, the Generation 7 Pokémon QR codes, the complete National Dex Gender, and Shiny specific hacked QR Codes downloaded. You can download them here or just scan every code from this imgur page.
This is a legal feature produced by Game Freak, not a hack or an illegal tool. If you want to explore and find what Alola has for you, maybe you should skip it, but if you want to go fast, you are more than welcome. Is this something you'll be indulging in?