A constant is used to define a fixed value which can be used during model. But is dynamic per configuration/environment. For integration purposes this can be used to define the base url. So that you can configure te acceptance environment to connect with the accp or test end point of the called rest api. And connect with the production endpoint in the production environment 

https://docs.mendix.com/refguide/constants​​​​​​​/

Map automatically is an quick option to match the mapping of json object items with selected mendix entity attributes. Just a nice feature of mendix.