It helped him think. That was always the answer he gave when John inquired, irritated that he was being kept up till a supposedly ridiculous hour, again. Really John you think that three 0'clock in the morning is ridiculous, honestly. He wasn't lying though, despite what people may think it really did help him think, just not in a way that anyone would have suspected coming from him. There were many reasons it was useful, it occupied his brain so that he didn't get too bored (even on a case there was still too little to keep him fully occupied), it gave him something to focus on so that his brain didn't go running off on tangents but most importantly and most unbelievably if you knew him, it gave him an outlet for his emotions so that he could continue to ignore them, shoving them into a small box in a dark hidden corner of his mind, never to influence, or distract him from, his work. He played on, a high, beautiful, haunting melody. The song of his violin expressing the emotions that he couldn't even admit he had, all of his need and all his worries that his John would get hurt during a case. As he played he realised, he could, did, love, and his newfound heart was beating.