I can't live without one, lol.
1) Convenience. You won't always have access to a telephone, such as your car or when you're stranded in a deserted road. Also, if you need to talk to someone, you can just take out your phone and press.
2) SMS is cheap, quick, and gets the job done. You don't always have to call to get the message across, such as "call me by 9am" or "meet you at the lobby".
3) It's more than just a phone. You can check email. You can manage your schedule & tasks. You can record people's names, address, contact numbers, birthdays, email ads, etc. You can access the internet & research or just browse. You can listen to music, watch videos, and play games. You can use it as a calculator and memo pad. You can also install special applications that help you with certain tasks, like my boss has a software that tracks his golf scores.
4) It's a camera, too. 8D
Not all those who wander are lost.