To understand why a reference model is important it is necessary to understand a little about the way standards are made and used.
When a new technical area is identified for standardization then experts are brought together to define the area. This results in a consensus of the general model applicable to the technical area. The general model describes all possible systems that could be built. An analysis of the general model will identify a number of components and interfaces between components. A series of design decisions are then made in choosing interfaces and components which will form the basis of a Reference Model. The Reference Model will prescribe all possible open systems in the technical area, which is a subset of all possible systems. The open systems will contain the identified components and will use the identified interfaces between the components. The functionality of the components, and their relationship, is set out in the reference model.
The interfaces represent the next step for standardization. Each interface becomes a specific standard. Work on the interfaces will be carried out in parallel to complete a set of standards. In the process of producing the standards a number of choices will be made in the design of the interfaces, which will further constrain the set of possible open systems. When the standards are completed they will represent a set of conformance rules such that any system supporting the standards will be a conformant system in the technical area.
The standards process is a progressive elimination of all possible systems down to a set of conformant systems. Each step involves making design decisions; and just as important, areas of design freedom for the next stage are identified. Eventually, design freedoms are left at the end of the standards process which represent aspects of the systems which are not essential for interworking; these areas can then be used by manufacturers to make unique products, and by end-users to create specific systems.
The Reference model is not part of the standard that describes conformant systems, therefore it is not a standard that systems can claim conformance to. The purpose of the reference model is translate the general model of the technical area into a framework which will enable its standardization. Once the individual standards have been identified, and their relationship established; work on them can proceed in parallel. This enables a technical area to be standardized in the fastest possible way.
In some cases the standards will define the products almost completely, this tends to be the case in OSI; where the communications design freedoms are very small. In other cases, such as management and security, the standards allow considerable design freedoms at a number of stages. Where decisions are made they are constrained to those aspects of interworking. Security and management have a big impact on the internal working of components and systems. The standards have a role to ensure that interworking can take place, and that when all the design freedoms have been taken up in an end-user system then that system will be secure and manageable as required by the end-user. Whereas OSI standards can specify the full details for communications, it is not possible for a security or management standard to fully specify a secure or manageable system.
Of course, one cannot mention standards without quoting the line attributed to Grace Hopper: "The great thing about standards is that there are so many to choose from".