Bike computers are much smaller than phones. They also have mounting backs so they can be secured with a bike mount in what requires a 90° turn meaning that even if you crash going 30 mph your bike computer probably is still going to be attached to the bike AND not broken unlike a larger phone. Also they can record in a fairly universal file format that you can take your data anywhere rather than being locked into a particular app.

Over the past 10 years it’s definitely gotten easier to use a phone and there’s nothing super wrong with it, but if youre serious about riding then getting a dedicated bike computer is nice since it’s going to be more specialized.

Kind of like you can get a multi-tool to do a lot of stuff but if you want a really good tool you might want to get the one specifically made for it.

A couple more answers:

- I’m not terrified to drop my bike with my bike computer attached.
- I’m comfortable leaving my bike computer mounted in the rain on a ride.

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Bike computers pre-date smart devices, but if I get in a crash I’d rather have my phone in my pocket than attached to my bike and possibly destroyed.

A bike computer is also smaller, lighter, uses way less power, is cheaper, and you don’t really have to manage it like you do a phone (i.e. take your phone out, start up the app, mount it, make sure you don’t run out of battery or you can’t navigate or communicate).