Not to mention, phones overheat in direct sun and stop working. Phones' capacitive touchscreens are also completely unusable in rain, contrast to the physical controls on a cycle computer.

Cycle computers are much lighter than phones generally. Their mounts are more robust and aerodynamic. If you crash, you're very likely to destroy your phone, whereas cycle computers are generally fine unless you directly bash them into a rock. Cycle computers have configurable high brightness LEDs for navigation cues and other training specific data display like HR or power.

Ok rant over. Mumble mumble Chesterson's gadget. If one doesn't know why it exists, maybe ask why...

Edit: phrasing for clarity