The memory cells in a solid-state drive can only be rewritten a limited number of times before they wear out. If the drive kept writing to the same cells, those would fail long before the rest. Wear-leveling spreads writes evenly across all cells so the whole drive ages together.
The drive's controller quietly relocates data to balance this, which is why a drive's internal layout rarely matches where the operating system thinks data sits. The result is a longer usable life and more consistent performance over time.