So a phone app does everything, but you're just worried about your expensive phone? Get another. Search "android prepaid phone" they're mostly around $30.
road cycling you go for 5 to 8 hours in bright sunlight with the screen at 100%, that battery is going to die very very quickly.
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