There are many reasons one should develop a series of drawings based on a single theme. Perhaps the most important reason is that the emphasis in thematic drawings falls on the execution of the art -- the process of creation rather than the product. Thematic drawings allow the artist to express more ideas and variations of those ideaas than would normally be possible in a single work.
Try the exercises below. While creating a series of works, you'll notice that thematic drawings help to create solidity within the expression of your ideas. You'll be able to see how many distinct compositional and stylistic variations can be used to portray one subject or meaning. You'll also see that thematic drawings allow you to get into your own work deeply, with more involvement and greater concentration.