In their heyday, EA controlled 1/2 of Concourse B (gates B-15 and higher, if I recall correctly) and all of Concourse C. They built this separate underground connecting tunnel to allow quicker transit from the high-end B gates to the high-end C gates. This saved connecting passengers the time and trouble of coming to the center of the Concourse to go down to the underground transportation mall to make the switch to the other Concourses.
"In this present crisis, government is not the solution to our problem - government IS the problem." - Ronald Reagan
Comments made here are my own and are not intended to represent the official position of Alaska Air Group