CodingKeys are created to map data between your model and the data. These are mostly created as an enum to easily map the data in a custom decoding or encoding logic. If you don't declare the keys yourself, the compiler will do this automatically for you. But as

The first chapter to customize your Codable (de)serialization is about the options of your selected Decoder and Enc​oder.

Codables are a very useful tool if you need to decode from or to different formats like JSON. Get your introduction into Codables here.