This PerfCounter64 counter reports a count of drop events, i.e. sub-intervals during which the port had to discard packets. An admin watches it because, unlike a generic error counter, this specific drop reason tells them which mechanism on the network (uplink) port is rejecting traffic, which narrows troubleshooting to that mechanism instead of the whole data path. A subscriber's support ticket about intermittent loss becomes a five-minute diagnosis, not a lengthy trace, once cmEthernetNetPortStatsESDE on network (uplink) port 17 shows 431 hits for the current interval while every other drop counter on the same port stays at zero.