Why draw when a photograph will suffice?

There are several reasons. 
Firstly, when one draws, one truly sees. The act of drawing demands a kind of attention that looking alone does not. The hand lingers, the eye slows down. Details and subtleties that might otherwise go unnoticed reveal themselves. Drawing is, in this sense, an act of seeing.
Secondly, drawing is a form of repose, particularly when there is music drifting gently in the background. Time seems to slow down, and the act itself becomes almost meditative. There is something deeply satisfying about becoming absorbed in a task that asks nothing more of the world than patience.
Thirdly, it is an exercise in problem-solving. One is continually searching for more elegant, more economical ways to accomplish difficult tasks. Each drawing becomes a small conversation between intention and invention.
And then there is the practical advantage of working digitally. Should someone wish to purchase a drawing to hang on his gargantuan lounge-room wall, the image can be upsized without sacrificing detail or sharpness. This is because a digital drawing is vector-based - mathematically described -  rather than pixel-based, with its fixed number of pixels.
But the real reason I draw is simpler. By choosing to make something difficult, I give the finished work a greater sense of permanence. The effort becomes part of the image. It is no longer merely something seen, but something patiently brought into being.