Mobile smartphones get their GPS signals from Assisted GPS (AGPS) chips. AGPS is great in populated areas, because is uses cellular networks, Wi-Fi, and other signals and hints to quickly get a rough idea of your location, which in turn helps the chip quickly find and connect with GPS satellites for more precise location data.
Unfortunately, this advantage is lost when you’re in rural areas, particularly if you lose your cell network connection. Then the weakness of the AGPS chips becomes apparent: dedicated GPS devices are much better at getting a precise location from GPS satellites.
Futhermore using an external unit for your GPS significantly increases the battery like expectancy of your smartphone or tablet