Why is there a diesel engine with a steam locomotive?

On certain trips, we use a diesel locomotive along with our steam locomotive. Due to the high overhead costs of running our trips, and the costs related to the use of the railway network, we have included the use of a diesel locomotive to ensure we can take a longer train with more passengers, and thus are able to keep the ticket prices within reasonable limits. On trips where we do not need to climb major gradients, we are able to operate with our steam locomotives on their own.