I take the flamethrower everywhere.
In Belize it revealed the true colors (read: more vivid colors) of the sponges and soft corals.
In Florida, we took them to peek into holes, look under ledges, etc.
Most recently In Hawaii, the water was often silty and not the best of viz - so they really came in handy. Again, holes, ledges and on deep dives.
Night dives is a no brainer. As were the 4th dive (the 4:00 PM dive) when the sun was getting low.
However, for us, signaling is the most essential use of the light. It amazes me how quickly and how intentionally buddies become separated in the tropics / clear water. People go in, and within MINUTES they're on opposite sides of the reefs - the man usually photographing, the woman usually touring slowly. Its been our experience that people just blow off the whole buddy thing in clear water.
Our lights keep Jaye and I in touch through the dive. When I'm shooting, I can look over and see her light and know she's near, and vice versa.
Whether in the tropics, in SoCal, in Monterey - anyplace. I can't imagine doing a dive anyplace without my light. I'll leave something else at home before I leave the light.
---
Ken