They are useful if you get lost and you have a map that gives UTM or lat/lon coordinates.
You can actually follow yourself on a map in real time display which allows planning turns, stops, etc if your unit has output capability.
You can answer the question "Are we there yet?"
You can find where you left your car.
You can set waypoints and see directions to get there.
You can record the exact path you took to get somewhere and play it back later for someone else to see.
You can prove how fast you were going when stopped by the cops.
You can use it as a compass (if you're moving).
When camping, you can use it to find your camp when you go on a hike or to town.
You can use it to estimate arrival times.
You can calibrate your speedometer.
You can use it as a speedometer if yours is missing or broken.
You can use it as a trip meter.
With the right database you can find restaurants and other things.
You can plan a trip ahead of time and the unit will even tell you where to turn.
If it is connected to a cell phone you can give the police your cell phone number and they can track your stolen vehicle (some extra cost accessories required).
You can locate the spot where the fish were biting.
You can amaze your friends.
You can find your altitude within a hundred feet.
You can find out exactly what time it is.
You can go back the same way you came easily.
You can amuse yourself on public transportation.
You can use it to learn something about navigation.
You can remember the location of that place you only go to once a year.
It can be a great navigation calculator. For example it will tell you how far apart two places are if you know their locations.
Many units can tell you when the sun will rise and set.