There are many benefits; but is all in the applications you buy. I have an iphone, and have downloaded many useful Apps.
A Recorder: Useful if you want to record an argument with someone, you can then press charges and use it in court.
A Digital Camera with video recording: Capture special moments, or if someone crashes into your car. (take their license picture, a picture of them, their license plate, their insurance)
Emails: Fast and better than having to go to your computer and reply.
text and picture messaging: Why would you call someone just to say "Hi"?
Google Maps: free, fast convenient, you can zoom in into a map and find your way around, or you can search form a store by typing "name zip code" SEARCH.
Music: Pandora, AOL radio.
Social Networking: Facebook, get in touch with your kids and you lost friends, unless of course you are not very sociable.
Games: for those boring moments and you just want to kill time.
Finances: There are apps such as Quicken, Quickbooks, mortgage calculators.
Sports: Apps to log miles via satellite, or GPS, you can then upload data to your computer and review your (pace, distance, HR, map your run, etc)
News, IM. You can have all of these for 1/2 the cost of a netbook or laptop, and have it all in the palm of your hand. And yes it costs $90-$100/month but unless you have no job and live with your parents, you can afford it. Just add apps and value to it.