A golf GPS device tells you how far you are from the green without needing a clear sightline to the flag. It knows the hole you are on and gives you front, middle and back distances the moment you look at it, which makes it the faster option when play is moving and the more useful one when the green is blind or the pin is tucked out of view.
The range splits into watches and handhelds. A watch stays on your wrist and reads at a glance. A handheld gives you a larger screen and more detail on the hole in front of you, including hazard and layup distances. Many models also track every shot you hit automatically, so you get performance data after the round without writing anything down.
If you want the exact distance to a specific target rather than to the green, a laser rangefinder is the other route. Those are in Golf Rangefinders.
