Dynamic wear leveling addresses the problem of a limited number of Erase cycles for a given block.

Recall that flash devices are consumable parts: You can perform a finite number of Erase cycles on each block before the block becomes bad (and must therefore be tagged by bad block management).

Here's why this could happen: the metadata update would erase any reference to the original data block(s), but would point to data block(s) on disk that were never filled with data.