Why? Because typically it offers the best kind of lighting for portraits: soft and directional, creating that Vermeer-like quality in an image: the deep reflective eyes and warm skin tones. Lovely.
This photograph required almost no post-processing because the lighting was right where it should be, to my mind anyway, and the effect--while possible to duplicate with studio lighting--is soooo easily done without studio lighting (a big plus in my book).
The subject here is positioned close to an open window, at a little more than 90 degree angle from the light (one can experiment with that), with the light either reflected or filtered by a diffuser of some kind (here, high clouds). I also positioned a reflector at camera right, to throw a little light back on the shadowed side of the face. The idea there is to keep some detail in the shadows and prevent the contrast from becoming too sharp.
So, the challenge for the day is to attempt a portrait using window light, if you haven't yet. With low light levels, you'll likely need to either adjust the ISO to a higher setting and/or brace the camera with a tripod (ideally) or against a wall or other object (less ideal, but still works).