J. S. Bach may have been the first to write harpsichord concertos, and even those concertos are more rewritten than written. With only two likely exceptions (including the Fifth Brandenburg), Bach’s 15 concertos for harpsichord are likely reworkings of compositions for other instruments. We don’t know why he made these arrangements, but they could have been training pieces for his own sons.
My own sense is that the history matters little now; just listen to the amazing sounds that are captured by this music, as performed by pianist Glenn Gould:
