This function allows DocumentsCorePack users to add a document to every created activity (only when using the file explorer or sharepoint setup).
For example, when you send a mail to a marketing lists, an activity will be created for each member of the marketing list.
This extra setting will ensure that the activity also has the document attached in CRM.
This article will show you how to work with this function.
The problem occurs because MS CRM is using an HttpModule for multitenancy scenarios. This http module is called with an anonymous user account. This is why it fails when trying to request, i.e. the metadata webservice.