It’s long been a preferred method of mine to use available light when possible. At first, this was because I was cheap — available light is already there, and you don’t have to buy any expensive flashes or lighting gear to use it. You just take pictures.
Most cameras include a flash built in. The problem with that flash is that it produces a very flat, uniform, familiar lighting pattern; on top of being ubiquitous, it’s really boring looking. It’s better than having a too-dark picture in many cases, but the appearance of it screams snapshot. You can’t see any shadows, so everything looks like it’s 2D.
The trick to using available lighting (at least indoors) is having camera gear that can actually work without a flash. You need a fast lens, or a relatively noise-free sensor (or, in the film era, fast film), and a steady hand. Outdoor shots with available light just make sense, and don’t require anything more than “a camera,” since there’s usually an abundance of light to work with. It’s those indoor shots that are tricky.
A picture like the one above, however, uses a trick that bridges between indoor and outdoor light, and takes advantage of both. I simply asked the woman to stand by the window. It was daytime, and overcast (because Seattle!), making for the world’s biggest softbox. If you look at the full size image, and look carefully at her right eye, you can actually see a reflection of the window frame.
It’s fun to use tricks like this to reduce the load (available light now means no heavy lighting gear to cart around), and you can also get some really good photos out of it.