[alexlrobertson]

Aside from the points other commenters have raised, one key feature of GPS head units like the Bolt is connectivity to sensors and accessories.

Sensors historically used the ANT+[1] wireless protocol which was the standard for monitoring sensors. Phones don't have ANT+ support and required an additional dongle.

Most sensors now support ANT+ and Bluetooth Smart which allows them to work with phones, but for the broadest accessory compatibility you need an ANT+ head unit.

Another point I'd raise is that the head unit allows me to ride with my phone off, silenced, on airplane mode, or even without my phone at all. The price point is significantly less than a phone so I'm also a lot less worried about damage or theft.

  [1]: https://en.wikipedia.org/wiki/ANT%2B