In my experience on the SP back then, they were used (like any other unit) to add their 1500 hp to the locomotive consist. Additionally, we had a lot of Alco 628's and 630's on the San Joaquin Division then and it was not uncommon to have one die on the road, leaving whatever EMD's we had to drag us to our terminal. Though I never worked around a K-M it's very likely that SP wanted to have at least one other working unit with a K-M in event of failure en route.