Albrecht Durer had studied the human body for years before finally mastering it within his artwork and drapery offered an additional challenge that required similar levels of practice. The way in which different cloth hangs from the human body makes it another aspect of portraiture that it much harder than it looks.
This study drawing appears to have been created just before another similar artwork, where the clothing is similar but where the model has been added. Therefore, perhaps he chose to create series of study drawings, adding more conplexity each time once he was happy with his work up to that point.
This knowledge would be used well beyond just this medium, where the use of line and shadow will continue into his paintings, particularly with regards watercolours. Many in that medium would have initial lines drawn in with colour filling regions created where they intersect each other. Considering how several drawings appear particularly similar, it is likely that the elderly man faetured elsewhere was modelling for this drawing too, though only to display the way in which the cloth hanged from his body.