When is it worth using a bike computer over a smartphone?
Your phone may be all you need until battery life, navigation or rough conditions start getting in the way.
The phone has an obvious cost advantage since you already paid for it. A good mount is much cheaper than a dedicated cycling computer and apps can be used for navigation and GPS ride recording.
Google Maps provides bicycle directions in areas where it supports them, but there's one annoying thing if you're going somewhere where reception is not great: cycling directions don't work offline, even if you've downloaded the map of the area. There are apps like Komoot and Ride with GPS that can do offline cycling navigation, but some features may require a subscription or map purchase.
An old GPS-equipped phone can also serve as a good bike computer. Fill it up with routes via Wi-Fi and leave your primary phone in a safer location, since GPS recording and downloaded navigation do not require an active SIM.
A smartwatch completes the picture by collecting heart rate and other fitness metrics during exercise, with the phone managing the map. Even the battery-life argument becomes less of a factor on the occasional long ride when you throw a small power bank in a bag.
There are still compromises. Your daily driver is left out on the ride to deal with rain, direct sunlight, crashes and whatever else the road or trail has in store. It's also consuming battery that you might need for calls or emergencies. And if you're uploading regular routes to a fitness service, check its privacy settings before casually publishing a very accurate map of where you leave home every morning.
