Consider the problem of many similar, but slightly different, clients wanting to talk to many similar, but slightly different, servers. In the general case you would need N(N-1)! connections. This does not scale very well with large systems.
The mediator sits in the middle. The clients talk to the mediator and the mediator talks to the servers. Only 2N interfaces need to be built and maintained. The mediator can be replicated as needed for performance reasons.
A mediator acts as the glue between many clients and many servers. See Figure 7 on page .