Lots of reasons..........Number one being that with the standalone GPS you have a standalone GPS------ you don't have to be fiddling around with the phone ,,bringing up the app--- then if someone calls ----you lose where you're at on the app..!!!!
A standalone GPS is just that -- standalone GPS.
Now ,,they're not always accurate --so you can use your smartphone GPS also to compare if you want to .
but A Standalone GPS Is basically like having automatic wipers --(mist )...you set it to the point where you're going to need it --(in the rain and you forget about it).. you don't have to be constantly adjusting-- that it's kind of like that.
You arent tied down to "your phone "...
I recently bought the "Good Sam " 7" screen one ...
Except for a few issues in the way its set up ( screen dim in bright sun ).
Couple other minor things ---- that they can FIX ....with a software update ........ I LOVE IT.