The idea for a rendering may be derived from anything, really - a flash of sunlight, the tilt of a head, a glass on the table, but the sketch is where it all begins, the point where inspiration meets artist. Gloriously free of the need to get it right, the sketch is where possibilities are explored, compositions are found and visions come to life. Sketches represent the art in its raw form.
Sketching is messy, unfinished and noncommittal. It allows you to have fun and think about things more unconventionally - without the expectation that it has to lead to anything at all. Sketches are a byproduct and castoffs of a thought in the process.
Year ago I would jump right into a rendering as soon as I got an idea. This would occasionally result in a decent rendering, but not as often as I'd have liked. When I started planning first and working things out in advance, my success rate tremendously went up. Sketching is unquestionably worth the time. Sketching has taught me that it's all about balance - between planning everything out and following that plan, but also allowing for hidden surprises and spontaneous happenings.