I've downloaded and uploaded many files which have arrived incorrectly over the internet, so yes, if you truly want to prevent bitrot, you do want to add error correction.

This is a transport problem, however. Why not calculate the error correction codes on the fly? Why wast disk blocks?

Because you want to guarantee a certain error margin, regardless of the error detection & retransmit/correct capability of whatever networks/file systems/media it traverses in the interval till you want to read it.