We have learnt that you will be able to use the Sony Smart Watch as a viewfinder and take pictures using the QX10 and QX100 lens cameras that Sony is all set to introduce at this years IFA trade show. The user manuals for these lens sized cameras have leaked online before launch. You can attach the QX10 and QX100 to any smartphone such as the Sony Xperia or the Apple iPhone and use the display of that phone as a viewfinder.
The Sony Smart Watch is already capable of acting as a remote viewfinder for your Xperia phone as showcased by Sony in a YouTube video. It can definitely come in handy when using your phone as a viewfinder is not easy- such as taking shots during a concert or using it for group shots as suggested in that YouTube video.