Theoretical capacity is not application throughput
A 1 Gbps link does not guarantee a 1 Gbps file transfer. Protocol overhead, RTT, loss, TCP congestion control, storage performance, CPU and competing traffic all matter. The efficiency field lets you state the throughput assumption instead of hiding it.
Bandwidth-delay product is shown separately because a high-bandwidth, high-latency path may require a large in-flight data window even when the line rate looks ample.