These days JSON is used a lot. For storing data, for storing settings, for describing other JSON files, and often for transporting information between server and client using DTOs (Data Transfer Objects).
When using an Azure Mobile App Service or ASP.NET Web API you will see that JSON is the default format to transport data. When running apps on a PCs with a fixed internet connection data size might not be a hot topic. But for apps on mobile devices, possibly using slow, limited, or expensive connections you want to save on the amount of data that is transferred.