If you buy a dedicated GPS, you're being ripped off, and that's it, period.
I was gonna say the only benefit of using a dedicated device is to be able to swap out batteries for AA, or AAA batteries which are sold everywhere, and still the most easy to find batteries, but like gun wharfman said, there's a load of all sorts of powerbanks to buy, and they're pretty cheap.
Actually, one of the benefits of using a dedicated gps is the screen still working when its raining, but meh, you can still use your phone in the rain, so they're still a rip off.