Drawing from Origin to Insertion
One of the best reasons for learning the origins and insertions of the muscles is that they provide well-located endpoints for lines on the figure. If you begin a figure drawing with a skeletal armature, you can then attach muscles to it at their proper points. This is especially useful when drawing the figure from memory.
You can make a competent figure drawing knowing only the general shapes and positions of the muscles. But when you learn the origins and insertions, figure drawing becomes a whole new ball game. This is because, firstly, your knowledge of anatomy will have reached a high level of detail, and that knowledge will provide a useful internal model for understanding the figure. Secondly, the attachments describe the poles of the axes through each muscle. Many muscles, particularly those on the limbs, are basically thick lines with bulges in the middle. Even broad ones like deltoid and gluteus maximus can be seen as having a primary direction to them once their attachments are known. You can then lay the muscles down on paper (or canvas or stone) with great confidence.
It happens often that the skilled artist will draw a muscle along its border from origin to insertion. Left alone, such lines have volume and liveliness, and the artist can efficiently build them into massive form.