You are correct to notice that an application has been created for GRUN enterprise. No this wasn't created by the app express. It has been added to indicate that you can create an enterprise application which could meet security and reusability requirements. In the practice application it supports the Node-Level Data Page D_RootProductCategory.
If your enterprise is to have multiple applications with reusable components in your enterprise that could be deployed separately then it may be wise to have an enterprise application. It isn’t required for all applications and business scenarios.