Why would you need truecrpyt if you could just use a bios password?
Because TrueCrypt is nice.
It lets you make mountable drives which you can triple encrypt, plus add password protection.
So, if you have a family member who is computer literate, they must have a copy of TrueCrypt, find the file the drive is hidden in and guess a password.
Its the paranoid solution to security.
Oh, and you can make hidden OS'es and junk, and its all free.