The PFD Manufacturer's Association has an FAQ page at their website that says:

Why do some PFDs have to be worn to meet the carriage requirement?
The Coast Guard requires that some Type V PFDs must be worn in order to meet the carriage requirements. By definition, a Type V PFD has some feature not found on other PFD type, that require special user knowledge about how to use the PFD. By requiring it to be worn, it is more likely that the user is more aware of the special features and knows how to use the PFD properly. Be sure to read the label so that you understand what the special features of that device are and know how to use it. In addition, wearing a PFD greatly increases your safety because you have it on when you need it.
http://www.pfdma.org/faq/default.aspx