Yaesu made a hand-held amateur radio, the VX-8D, that had a clunky add-on GPS unit. Users demanded GPS built into the radio itself. As a response, Yaesu created an additional version with GPS built in, the VX-8G. People who value the GPS functionality are buying the 8D with the add-on, because the 8G's built-in is slow to acquire satellites. I don't pretend to know the science behind it all, but perhaps that is why Canon went with an add-on.