The help desk provides a service - a service to other parts of the organization or perhaps to external customers and external clients.
As such, it is extremely important that each party understands the nature of the service, the expected level of the service and the agreed responsibilities of each party.
These issues are usually dealt with via a service level agreement (sometimes called an SLA). This defines all these matters and many others, and is the core basis of understanding between the requisite parties. In short, it is an EXTREMELY important document indeed.
Help desk managers therefore should not only ensure that a service level agreement is in place for important systems , but that they FULLY understand their defined responsibilities. This is surely a pre-requisite to effective delivery of that service: understand exactly what is to be delivered, in what volume, when and how. Also, what are the implications (penalties, if any) of failure to meet the SLA? The list is extensive.
If no service level agreement exists, it is highly recommended that one is considered.