Yes...It may happen that you are forced by somebody to reveal the password to an encrypted volume....There are many situations where you cannot refuse to reveal the password (for example, the feds are trying to get your digital blackbook). Using a so-called hidden volume allows you to solve such situations without revealing the password to your volume.
So like you accidentally tagged the house of a Mexican drug cartel member and he is holding you hostage and demanding the password to the encrypted files on your usb key.
Yes and if you use a hidden volume there is no way for them to know if you even have hidden files or not. Here check it out. https://www.truecrypt71a.com/documentation/plausible-deniability/hidden-volume/
So what you do is put some bullshit files on it like pictures of you fucking Fagaroni's mom in the ass that you give up the password for and just deny that you even have any hidden files on it and there's nothing they can do.