The importance of sketching has always been stressed by the masters. Some may feel that when painting from photographs, its not really necessary to do a preliminary sketch. I, for one, used to feel the same. It may be okay in case of simple subjects but when some subjects are really complicated, you need to simplify, or when you feel that some colors may be changed or pushed to create a mood, then a sketch or a little study really helps.
Anyway…doing a small sketch is always so much fun J