A. There may be environments where a drive letter should be hidden from a user because you don't want them using it. For example, you might have a drive where you store a certain type of system cache, or a Citrix Virtual Desktop Infrastructure write-cache.
You can hide drives using Group Policy. Go to User Configuration, Policies, Administrative Templates, Windows Components, Windows Explorer, Hide these specified drives in My Computer. By default, you have options to hide the A, B, C, and D drives, as shown here.
If you want to hide additional drives, you can modify the system.adm file, as detailed on this Microsoft site.