Glancing through my apps to see what I use it for:
1. Communication (calling, texting, emailing, and social media)
2. Banking (tracking, banking, credit card, Paypal, Square)
3. Music & audio (Spotify, Downcast podcasts, TED talks, Shazam for ID'ing music)
4. Child entertainment (games & Netflix for long car rides & stuff, interactive books, etc.)
5. Smarthome & security (Wink home automation & Foscam webcam)
6. 2-factor authentication to a bunch of websites for security (Google Authentication app)
7. Work stuff (VPN, RDP, emergency screenshare)
8. Traffic (GPS & Traffic View for traffic cams)
9. Looking stuff up with Google when I have a question
10. Photo camera & video camera (it's my primary camera)
11. Jotting down notes (Evernote)
12. Shopping (Amazon, Craigslist, eBay, grocery store delivery, Red Laser for finding stuff IRL cheaper online)
13. Fitness tracking (hopefully will get more into that with a FitBit-esqe device soon)
14. Entertainment when I'm bored or waiting in line somewhere (games, Youtube, Vimeo)
15. Alarms & scheduling (calendar, morning alarm to wakeup, eating alarm for bodybuilding meals)
Pretty much indispensable...puts everything in one place. I carry a wallet, phone, and keys. With Apple's upcoming payment system & a wallet case for my driver's license, and Viper's SmartStart system for the car (& Wink's home automation system for the front door lock), I could probably get away with just carrying my phone only.