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Here is the official word from Microsoft TechNet, The C:\Users folder is shared when any sub folders from C:\Users\<username>\ is shared. The article is stale and mostly unhelpful, but worth knowing to complete the story.
Shared folders are used to provide network users with access to files and application resources on the network. When a folder is shared, users can connect to the folder over the network and gain access to the files that it contains. However, to gain access to the files, users must have permissions to access the shared folders. A shared folder can contain applications, data, or a user's personal data, called a home folder.