It has the additional responsibility of providing a global point of access to the instantiated class. One obvious place for using the Singleton would be in storing application-state data, so that when a user starts an application it has the same visual appearance and data loaded as the last time it was shut down. This pattern can be used in many different contexts at the same time. The authors take great pains to make each pattern as understandable as possible. Unified Modeling Language.
Design patterns are of interest to developers, but probably no one else.
The UML is different. Not only can this tool be used to help us in modeling our software, it can also be used to communicate design decisions to others - our managers, clients, colleagues, etc. This should be obvious since models provide a means of connecting an application with the specific domain in which it is to operate. And well it should be. UML is simply an extension of this approach.