Communicate by phone, message, Email.
Find your way by any means of transport and on foot especially in unfamiliar locations.
Use safety features that enable you to call for help and to be tracked by your friends and loved ones when you are out and about.
Leave your credit cards and important documents such as driving licence at home and have working copies on your phone.
Have access to all your computer files whilst out and about.
Get up to date information on the weather.
Find local shops and services.
Book tickets and carry copies in your phone.
Take instant photos and videos not only of things you want to remember but also of events that need to be captured for evidence.
Travel out of your home with all of your entertainment media such as books, music, movies, radio.
The best reason of all is that in this one small device you can always have with you everything you need to function in your daily life.