Bach, Handel and many other lesser composers recycled their own music and the music of others via transcriptions and arrangement. When the workday and responsibilities of a Kappelmeister such as Bach are looked at with choirs to train and rehearse, instrumentalists to train and rehearse, and music to be prepared for every church service in many different churches (with each demanding their own music), it is no wonder that even Bach himself had the inspiration to write new music worked right out of him.
Add to all of that, beginning in 1729 Bach was appointed director of the weekly Collegium Musicum concerts in Liepzig, which also demanded new or refurbished music for each concert. It was for these concerts that Bach reused some of his music, pouring them into new forms and instruments. The Harpsichord concerto No. 5 is one of those hybrid pieces. Bach used the outer movements of a violin concerto and a movement from an oboe concerto for the second movement. He also used the second movement melody in a cantata.
The concerto is in three movements:
I. Allegro moderato - The first movement is in ritornello form and is full of rhythmic energy and seriousness.
II. Largo - A cantabile song that almost seems endless. Bach seems to have been quite fond of this melody considering how many times he used it.
III. Presto - The feelings and mood of the first movement return along with some echo effects between soloist and orchestra.
Bach may have reused his music, but his genius and creativity always added something to the original that makes the transcription worth hearing. In the keyboard transcriptions of his violin concertos and other works Bach gives a new independence to the left hand that used to be relegated with doubling the bass instrument and filling in the harmonies. For that and other innovations, Bach stands at the beginning of the formation of the standard keyboard concerto as practiced by Beethoven and Mozart. Bach was so skillful in his transcriptions that it can be very difficult to think that the music wasn't originally written for the instrument. Bach was a genius by most stretches of the imagination, but he was also a master craftsman.