You need a bike computer if you're really into cycling, especially at a competitive level, or you regularly do long training rides and want a complete report on your rides because that's exactly what bike computers are for. However, the device in your hand (your phone) or the gadget on your wrist (if it's a fitness tracker) can also provide feedback. Both of those options may save you some money and provide you with all the information you need.
Using Your Smartphone as a Bike ComputerA third-party cycling app can be installed on almost any GPS-enabled phone. Strava and Mapmyride are two well-known options. These apps provide performance data, help you navigate, and then allow you to download your data for analysis later. You can also share data and routes with friends and compare your rides with others in the online community. The app itself is typically free, though you are limited to basic features like logging mileage and routes. You need a subscription to access a more robust set of features.
Many riders also use phone apps with their bike computer or fitness tracker, so it is worth your time to look into several cycling apps. Check out the fitness-tracker and bike-computer apps, which provide much of the same functionality (as well as device setup and customization).
If you decide an app-enabled phone is what you need, you will need a bike phone mount to mount your phone to your bike's handlebars.
One argument against using a phone as a bike computer is that it consumes a lot of battery power. As phone batteries improve, this becomes less of an issue, but batteries in bike computers and fitness trackers will almost certainly last longer. For some, preserving the battery power of their phones is a safety concern. In the event of a collision or mechanical failure, they prefer to minimize the risk of losing the ability to call for help.
Using a Fitness Tracker as a Bike Computer
Many GPS-enabled fitness trackers offer cycling modes that provide performance feedback and navigation assistance. It's a great option if you enjoy many activities and nerd out on each one. Besides, it will already be on your wrist, so you won't need a handlebar mount. (That also means you won't have a navigation screen there.)
Using a Dedicated Bike Computer
While versatility is crucial, having purpose-built technology like a dedicated bike computer will give you the most accurate data and the broadest range of metrics. Furthermore, many offer large customizable displays and extended battery life for long rides (or races). With any high-tech product, paying more gets you more powerful processing, a more sophisticated user interface, and more features.