Thick clouds.
Yes, if the sun is up you don’t need one. However, when you find yourself navigating a couple miles in unfamiliar territory in a driving rain with the sun setting you’ll appreciate a compass. I now have one tied to my pack and a small one on my watchband.
+1!
What's this "Sun" you speak of?
Combine no sun with flat terrain and unbroken forest and you've got the recipe to get a might-bit turned around.
