The SLA pulls together information on all of the contracted services and their agreed-upon expected reliability into a single document. They clearly state metrics, responsibilities, and expectations so that, in the event of issues with the service, neither party can plead ignorance. It ensures both sides have the same understanding of requirements.
Any significant contract without an associated SLA is open to deliberate or inadvertent misinterpretation. The SLA protects both parties in the agreement.