The Service Level Agreement (SLA) is a tool to define expectations for the customer as well as the expectations of the IT organization. An effective SLA is generally written for a service which means it bundles all of the hardware, software, and IT processes required for an end user to perform a business function that ultimately allows the business to meet their objectives.
IT must help the business understand the cost of service to ensure they can make informed decisions about the required performance as well as the response from the IT organization. When open conversations occur between the organizations, the business is involved in the decision making process. They understand the cost variables associated with the architecture and how it impacts their day to day operations.
The IT service is delivered in accordance with the service level agreement that was negotiated however, the involvement of the business at crucial decision points, encourages trust and partnering between the organizations.