This counter tracks the cumulative megabytes read from this specific logical unit. Comparing how fast this counter grows against the write-megabytes counter for the same LU tells an admin whether that particular volume's workload is read-heavy or write-heavy, which matters for cache sizing and tiering decisions on that volume. If this counter increases by roughly 500,000 during an overnight window, that is about 500 GB read from the LUN, consistent with a full backup pass.