People mount phones to their handlebars for GPS navigation and to use as speedometers on bikes that did not come with one from the factory (e.g. dirt bike conversions in places where that is legal)
dharmab is correct, the use on dirt bike handles is common, doing things a GoPro can’t.
On the camera side, a GoPro is fixed-focus and has a fixed aperture of (usually) f/2.8 and varies the exposure, shutter speed and frame rate unless set. There is no autofocus to damage. However, there are also no navigation apps for GoPro.