Designing the System Under Test (SUT) for testability is important for a good test automation...

Observability: The SUT needs to provide interface that give insight into the system.

Re-useability: The code written for the SUT must be re-useable for other similar system.

Clearly defined architecture: The SUT Architecture needs to provide clear and understandable interfaces giving control and visibility on all test levels.

Control: the SUT needs to provide interfaces that can be used to perform actions on SUT.