Explain why anyone would want to create a veracrypt hidden volume?

You have to create a veracrypt hidden volume for two related reasons.

If you are arrested by the government and they suspect that some of the data in the encrypted volume is evidence, you can be forced to provide the password. Since TrueCrypt advertises its ability to create a hidden volume and encourages its use (and that's stating it mildly) the government is right to assume that a hidden volume exists and can force you to provide the key. If no hidden volume exists, the burden of proof rests with you: you have to somehow prove that despite everything you didn't create a hidden volume. So you have to make sure that such a volume exists, so you can provide the password. As a corollary, make sure it's an easy password and fill up the hidden volume so no empty space remains.Non-government attackers similarly will suspect the presence of a hidden volume. Not bound by rules and regulations, they will just beat you until you provide the password, so you better make sure there's a password to provide.