The stretch helps make the muscle grow due to increased microtrauma caused while being elongated under load. That's my basic understanding through vicious, anyway. He or one of the more intelligent forum-goers could give you a more detailed explanation.
In any case, they are a very stubborn muscle, comprised of mainly slow-twitch fibers. They're used daily, so it's no wonder people have trouble growing them. The stretch is to increase the microtauma. It's often recommended you do them on a 2" wooden block, your heels hanging off the back. Rather than going flat-footed on a normal floor, you let your heels dip back, while pulling your toes up, so to speak. In theory, this should place them under greater stress and cause more growth.
I would not hold the stretch for fifteen seconds. Something like 3-5 seconds (no quick counting, as many do) should suffice. Higher reps are often used, as well as greater jumps in loads. People often have trouble staying ahead of the RBE with this muscle group.
On a final note, vicious often advises donkey calf raises. It's a wonderful stretch-point movement for the calves.
