Sticking with the OP question... "Why buy a GPS.. when I can I use a phone app?"

I see most of the pros as external; Weather proof, compactness, display visibility, battery life, less expense in case of crash... though the weight of those factors is debatable as I have no problems with any of those using a cell.

And I have yet to see a critical difference in data measurement accuracy between cell apps (using GPS only) and a dedicated computer. Yes, there were occasional spikes in realtime speed using GPS only. That went away once I installed the DT Sensor and connected to the app. But, the bottomline is my ride result details always showed the same end data (Dist, Avg Speed, Elevation Gain, etc)on Strava as my friends using the computers.

I wouldn't tell anyone NOT to buy a dedicated computer unit.. but, I would offer to try out a few of the free cell apps using GPS as a starting point. Wahoo is pretty slick with/without using sensors. Learn how to sync and upload to other apps like Strava. It'll provide a baseline for making a better decision of what features you'll want when the time comes to move up to dedicated computer.