Larry Osterman, da Microsoft, responde a essa pergunta aqui:
Out-of-the box, a Windows system automatically shares the root of every hard drive on the machine as $ (so you get C$, D$, A$, etc).
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the development lead for Lan Manager (Russ (Ralph) Ryan) needed to change a config file on the LanMan server that hosted the source code for the Lan Manager product. And he realized that none of the file shares on the machine allowed access to the root directory of the server! He couldn't add a new share remotely, because the UI for adding file shares required that you navigate through a tree view of the disk - and since the root wasn't shared, he could only add shares that lived under the directories that were already shared.
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Nowadays, with RDP and other more enhanced remote administration tools, it's less critical, but there are a boatload of products that rely on the feature.