The first version evolves over time to new requirements, which introduces cruft as you can never prepare your code to be extended to every new requirement. Subs

The first version evolves over time to new requirements, which introduces cruft as you can never prepare your code to be extended to every new requirement. Subsequent versions/rewrites benefit from that history. Using the existing version as a complete spec, and the history of evolutions as a clue to how to set up extendability, gives re-writes a distinct advantage in producing better code.