If your friends don’t have a data plan and you do, your smartphone can give them internet access. You can become a source of hotspot for other people.
You can use the Smart Traveler app. In this you will get alerts and warnings which can be useful for you. You will be able to find out from this that how safe the place you are going is. You can know what is the condition of earthquake or any other natural calamity there.
As long as you have location enabled on your smartphone, you can go anywhere without Wi-Fi or Google Maps. You don’t even have to use data for this. You can use Maps.me. Through this you can see offline maps.
You can use the smartphone as your personal assistant. You phone is best for saving reservation details of your flight tickets, theater tickets, museum, restaurant etc. You can organize all this in apps like TripCase and TripIt.
With the DuoLingo app, you can learn the language before you travel. Also, if you are going to another country and you do not know the language there, then you can use the phone as a translator.