A service catalog serves two major purposes: informing your customers (IT resource consumers) of service capabilities and offerings, and abstracting those offerings from the underlying technology. Without a properly designed catalog, the technology is tied too closely to the service provided. For example, a user requiring high-performance storage may state a requirement for Fibre Channel storage on FC disks using a specific array. You may be able to deliver the same performance/reliability at a lower cost with other technology, but the user's request for a Fibre Channel-attached array is what they know or are used to.
A very basic service catalog will abstract the underlying technologies and provide service-level options such as silver, gold and platinum with definitions of what those provide in latency and reliability terms. This allows you to change the underlying infrastructure based on cost, performance and technology improvements without changing the way services are requested.
The catalog should be designed so that the desired service is emphasized while abstracting the technology used to provide that service. Take mobile security as an example. A service catalog could be designed with a list of software and security options to lock down a mobile device, allowing the customer to choose applicable requirements. While this would work, it requires additional knowledge from the resource consumer and complicates changing out underlying products. Instead, a catalog could offer the option for mobile device security and implement all required products and services on a uniform basis.
With a well-defined catalog in place, it's time to apply costing to the offered services. The service consumers should be aware of the cost differences of each option, even if the departments are not directly charged back. This provides not only a means for those consumers to make cost-effective decisions, but also a means to track back costs to departments, which helps in budget discussions.
Providing visibility into the services being offered and the costs of those services assists in allowing your end users to streamline what they consume and provides justification from big spenders when IT purchase requests must be made for capacity increases.
An additional benefit of the service catalog is the visibility it provides to IT consumers. It allows the consumers to know of all of the offerings and capabilities provided. Especially in large organizations, departments may not be aware of one another's tools and applications. Providing visibility via a service catalog can allow departments to identify overlapping services that can benefit the business or mission.
Service catalogs are a valuable tool of a private cloud and are key to overall success. They are another step of abstraction between the technological widgets and the business demands that allow a private cloud to thrive.
Disclaimer: In my primary role I work with several products and vendors that provide the software and services mentioned here. This article is not an endorsement of those, or any, products or vendors.