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.
You could use a phone (I did for a bit) but the experience is worse in every single dimension. Until, of course, you get really lost and need Google maps...
Actually, I have also found it extremely useful when navigating unmarked trail races. It's small and light enough to keep in your pocket and the mapping beats anything on a GPS watch. I use it to track my progress too, no problems with elevation for me but there isn't much of that in the UK anyway.
I used a Garmin for about 5 years but I got the Bolt last Summer and I have found it considerably more reliable. The worldwide maps are great too. However, on the basis of the complete disrespect for OSS I will stop recommending it to others and I won't be buying another. Shame.
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%2BThe Elemnt also comes with a bike mount so I can just take a quick glance down for directions instead of having to take my phone out of my pocket. I could just get a cell phone mount, but I haven't gotten that far yet.
Also, a dynamo could work - but that would add additional friction and make it harder to cycle.
But above all else, riding with a phone out on your bars is just asking for trouble. The Bolt is so easy and convenient to interact with since it still use tactile buttons. When it's sub 40s and I'm cold, or 85+ and sweating, the last thing I want to do is fight with a touch screen.
You could even have it charged from the dynamo.