Cross operating system compatibility. I can put something (like my tax info) on a true crypt disk on my Mac, and then email it to my mom (an accountant) who can open it on her windows PC.
Which leads to another benefit, my mom is no system administrator, but she can open a file, enter a password, and double click the file within.
Further more, if I want to deal with it - I can put it on my Linux machines.
Finally, if a technician needs to fiddle with the system, I can unmount the drives and let them in with (less) worry about what they may find. (Tend to deal with health care information.) In other words, I can compartmentalize who can see what.