I use my phone's flashlight a good bit, it is very useful. That said, if I was going on some away mission where lighting was going to be questionable, I'd probably bring some kind of dedicated lighting equipment like a flashlight.

My flashlights have adjustable beams. They throw out many times more light than my phone's flashlight. Using the dedicated flashlight gives a way better lighting experience than using my phone's flashlight. But it is like cameras, the best flashlight is the one on you.