I work for a mid size telco. VoIP has been the predominate method for voice communications going on 10+ years it was just behind the scenes.

I use the Realcalc scientific calculator app at least 5 times a week. As others have mentioned GPS functionality for driving is great. I also use my phone’s GPS to get waypoints for the location of fixtures for my job as a telecommunications facilities designer.

Having a high quality camera is also handy for work and personal stuff.