The only reason Workbench needs to be installed on any the DataStage Engine (indeed, the only reason it needs to be installed on any DataStage host) is so that it has access to the same file system used by the MettleCI Unit Test Harness.
It's the file system that acts as the communication mechanism between Workbench and the Unit Test Harness. Workbench supplies the Unit Test Harness with test specifications and associated test data, and the Unit Test Harness provides unit test results in return. The Unit Test Harness can also supply test data when running a job in interception mode.
The Unit Test Harness needs to be installed on the Engine tier due to the nature of its tight integration with the DataStage job execution mechanism.