If the interface is running duplex, which many, if not all, WAN interfaces do, the total interface load would be the aggregate of ingress and egress. However, often we're more concerned about just ingress or egress loads. The reason for the latter, once your ingress or egress starts to hit a "high" load percentage, it generally means your almost out of ingress or egress bandwidth, and even before you totally run out of ingress or egress bandwidth, traffic might be impacted by a "high" load. But, if you have a, say, 50% load for ingress and egress combined, what does it mean? It might be only 25% for both ingress and egress, or perhaps 100% load for just ingress or egress. The former, often, a non-issue while that latter often is.
