The Master-Slave pattern is often used for multi-threaded applications in which many instances of the same problem must be solved. (Travelling Salesman Problem, for example.) The master creates and launches slaves to solve these instances in "parallel". When all of the slaves have finished, the master harvests the results.
Master-Slave pattern is also used for user interfaces and servers. In both cases the master listens for commands coming either from the user or from clients. When a command is received, a slave is launched to execute the command while the master resumes listening for more commands (such as the "suspend the last command" command.)
In this example we have refined the Master-Slave pattern with the Factory pattern.