USB phones are popular for two main reasons, portability and price. They're not really phones as much as they're phone-shaped headsets that work with your computer's softphone. Because there's no real internal phone brain in them, they can be manufactured cheaply and can be kept small and portable.
USB phones are excellent for use as speakerphones. There are several good, small, high-quality speakerphones available that are perfect for that impromptu conference call at the office, or to get the whole family in on the conversation with relatives. We even use them here for our own conference calls.