When an application writes data, the changes are first stored in the cache, and not until the cache line is evicted is the entire cache line written back to memory. If only part of the cache line is written to, then precious bandwidth is consumed to write-back data that has not changed.

Write-back utilization expresses how much of the cache line that has been updated. You should strive to get a 100% write-back utilization to minimize the amount of wasted bandwidth.