Quite by accident I found out how much better a phone running the strava app is when comparing it to my garmin. For one thing the live segments display is much better, especially when you stumble upon a popular one and it counts you down and then shows a graphic of your progress against the kom holder in a clear, big and colourfull manor.why would I use a phone when I have a GPS? it helps understanding how the assist works, you get a feel for how hard the motor push, really interesting to see, on my fatbike not much assist is needed going straight on asphalt, but when I bike just beside the road the assist level doubles. That is nice information to have I think, especially if you're going on long rides. But not necessary by any means of course
Actually, if you're having a Garmin beside the Garmin display is not necessary at all. You have the power meter on the battery pack. You have +/- buttons. The display could be removed completely. It would be cool if there was such a version and the Bosch system sending the data through ANT+ to your Garmin instead
Strava routes also display well, and of course text and call alerts (now available on high end garmins) are readily available.
I used a frame bag with an old phone in an insert protected by a plastic sleeve and given the choice would choose a phone.
With regard to the power graphic, I see it and understand what it's telling me but to influence it I need to peddle harder or not, but to what end? I know when I am using power because I can hear it.