When I was in college, one of the biggest lessons I took away was the importance of working in a series. Throughout the years I have learned that this is a necessity for me. When I draw or paint a subject more than once:
a) it helps me to understand the form and gives me a greater knowledge of drawing
b) when I see the pieces as a whole, it's always a stronger representation of the work
c) there is a greater sense of accomplishment and improvement
d) I have a better grasp of colour and space
e) I can work through problems and find interesting solutions
f) over time there is a better grasp of my medium of choice
And I have noticed that it doesn't matter the media or substrate as it can be anything from paper and pen to canvas and paint. I have found this process to be invaluable in my journey as an artist and highly recommend it.