When my car's GPS took me miles down a road that was washed out on a trip hundreds of miles from my home, Google Navigation on my smartphone suggested a different route that got me safely to my destination.
It's a flexible and reliable alarm and kitchen timer.
I can read novels comfortably in bed on my smartphone (none of my several laptops are very ergonomic for this purpose).
I can map my bike rides and get stats on min/max/average speed, elevation, etc. (all the while using it as a music player).
It lets me easily maintain multiple shopping lists.
I can tune my guitar with it.
It functions as a real flashlight (my phone has a very bright LED).
My smartphone is no replacement for a computer, but it's better at these tasks than any computer I own.