[pessimizer]

Encrypted volumes look random. There's no way of proving whether or not you have something you can't decrypt. Hence the need to have some innocent volume to decrypt as a tool in the argument to convince an interrogator that you have nothing left to hide.

[loup-vaillant]

Better yet, fill every hard drive with random junk before formatting and selling them. If everyone has random data in their free space, it won't even look suspicious.