Most often these work units are subsets of the application data, but they could also represent independent tasks to be performed on a single set of data.
Some applications make multiple passes over the application data before determining a final result.
One example of where this structure can be very useful is when a set of machines is accessible only through a firewall. In this case, responsibility for the machines behind the firewall can be given to a submaster running on the firewall, eliminating any need for the master process to communicate directly with the protected machines.