Why would an artist work from a painting completed hundreds of years earlier by another artist? It might be that one wants to learn how to craft like a master, but if you were Pablo Picasso it would be to make it your own. Picasso saw art as invention and many times referred to existing masterpieces to re-work the subject matter in his own imagination, to his own liking.
An article covering the release of Picasso's Variations on the Masters: Confrontations with the Past, by Susan Galassi in the Smithsonian Magazine, September 1997, brought attention to Picasso's work from Las Meninas, Velázquez's masterpiece from 1656.
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In four short months, at the age of 76 in 1957, Picasso made 45 variations from the painting, 58 studies total, including isolated features, treating the process as an inventive constantly changing experience as he made the work his own. (That's roughly 1 painting/drawing every other day.) (See on Wikipedia).
Throughout the creative process artwork changes often and not always as expected, a matter of fact which can frustrate a less experienced artist until he or she overcomes the need to be right or worse - finished, and stops seeing an unfinished composition as not good enough. Taking a second look at what has gone before with new eyes to explore different ways of doing things was foundational to the innovations that were intrinsic to Picasso's work, and his contribution as an artist.
In the studio, especially through the abstract method, but through every sort of approach and media, exploring and inventing is what our work is all about and we get there through confronting not just the past, but learning to balance the inner critic with the inner artist as we move ahead, pause to see, and continue. That is the secret to making the creative process truly satisfying, then and now.
Consider practicing a little art yoga at The HeartStudio.