I don't have cell service where I hunt in the mountains in Central PA, so that is why there is a need for the GPS.
I'm not sure about other apps like Onyx or others but I use Backcountry navigator on my Samsung. Doesn't matter if I have cell service or not, links right up with a satellite. I did pay the one time fee of 9.99 years ago for the Backcountry App but I'm sure there are a whole lot of free ones out there.
