The apps on your phone do not identify truck prohibited routes, low overpasses, weight restrictions, etc, - they are intended for use by passenger cars, not 80,000 lb commercial vehicles that are often 13' 6" in height or more. You don't want to be this person...
A current Rand McNally Motor Carriers Atlas is a requirement for commercial drivers for the information in it, identification of legal routes, restrictions to commercial vehicles, etc. You'll need this for trip planning, besides navigation.
GPSs are handy for enroute navigating, but can't be trusted 100%. They are only as good as the programming and databases that are used. The situation on the ground does change - there is new road construction, and local governments don't always communicate travel restrictions in a timely manner. That being said, finding a poorly marked road at night is a lot easier with one than without. When dispatch calls and wants a firm estimated arrival time, a GPS gives you the distance to be travelled with no guessing involved. It's a handy tool.
A trucker's GPS allows you to enter the height, length and weight of your vehicle. The GPS will route you away from low bridges or roads that your truck should avoid. Your cell phone will often charge for data use using the GPS function. The trucker's gps will tell you about all truckstops and weigh stations along the way.
In short, your life will be a lot easier with a GPS unit.