The flow of drapery can be hard to depict accurately and required as much attention as a facial portrait or landscape scene. There is a clear focus on the clothing within this study, with very little detail afforded to any other areas of the sketch.
Michelangelo would often draw a very faint, rough outline for the other elements of the composition before attacking his main focus. Building an approximate female as seen here would help him to layout her clothing more accurately.
When you compare this drawing to Michelangelo's completed fresco of The Erythraean Sibyl you will see her pose remained very much the same. The artist merely added more detail to her clothing, her figure and also some background scenery to help it fit with the other elements of fresco which were together creating a larger overall composition.
This drawing has a loose dating of 1508-1512 and features brown wash and pen with dark brown ink over a black chalk underdrawing. The Erythraen Sybil would appear in the Sistine Chapel as a fresco element in one of the world's most extraordinary artistic achievements.
It is probable that the drawing was completed just prior to the artist completing the final fresco although, like much of Michelangelo's career, there are still plenty of questions left unanswered because of the sheer amount of time that has passed since his work first took hold all those years ago.