Battery life mostly.

I've used the device for 4 hours this week and the battery dipped from ~50% to ~40% (that is witout backlight and other variables which might consumer more energy)

I believe they are also weather proof (mine didnt mind some water splashed onto it).

A Dynamo to power my phone would unnessecarily add another drag coefficient and slow me down.

It is also claimed that the device is more aerodynamic than the competing devices from GARMIN and also a phone based solution.

The device also has some very nice LED indicators for metrics like heart-rate (blue when resting, green in sweetspot, orange/red when burning matches and dipping into non-aerobic zone).

I could go on but this list covers most of the reasons I bought the unit.