It has been the practice since the Renaissance for an artist to study the works of the masters. An artist's education is constantly evolving. Improvement of draftsmanship/drawing and painting skills have not only been studied through working directly from life but also through copying/interpreting existing artworks. Hence an artist discovers the tricks of the trade so to speak and learns how he can better translate his own personal vision.
This tradition has been carried over from the apprenticeships to the masters of the Renaissance, to the Beaux-Arts education of the 19th century up to and including the education of today. Student artists have been required to visit museums, specifically the drawing/print rooms as well as the galleries. Here they can directly access the collections where they can draw and/or paint from a broad variety of works, thus learning as the apprentices before them.
The works that have resulted whether direct copies or interpretations have been noted as "d’aprés." In other words "after" or “"in the style of" or "in the manner of" or "in the fashion of", etc.