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

Thanks, this was actually quite helpful. I’m just starting out, and had been wondering if I should get one.

To save me some buck I strapped an old sport watch to the bike, it can connect to everything, last days on a single charge and it has physical buttons. Just as good.

Ooh yes, you can even get purpose made handlebar mounts for sport watches. Good point!

If the expense is an issue, try a used one. My cycle computer was released several years ago (wahoo wfcc4 (2019)) and is still great.

they asked for the cheapest vario beeper

Varia alerts don't utilize the screen, they are just buzzes and beeps. A phone will last for days with screen off, especially with no sim card. They asked for the cheapest option.

Varia will show you how many vehicles are behind you and an indication of how far away they are. You're missing a lot without a screen.