Based on my reading, the app web is really for SharePoint hosted app to host all those client files and it could be used for storage for SharePoint hosted app. For provider hosted app, it is hosted at remote web and it could use CSOM to access SharePoint, so there is no app web here. Right now, we are building a provider host app with SharePoint App Parts and we would like to store some data at SharePoint and we are looking into if we could store at app web if there is such app web.
However it seems that I could not get a definitive answer whether there is an app web for provider hosted app or not and how to access it based on online documentation.
Thanks in advance for your help.