As a SafetyWallet member, can't I just apply one category to a task?

Unfortunately, not! In most cases, any task you perform in your workplace will involve a combination of the category types. They dovetail and overlap one another, providing a comprehensive set of controls for any particular task. So, you may need to put an extractor (engineering control) in the workplace to remove any dust created during a task but at the same time, include a safe operating procedure (administrative control) for the task which includes operation of the extractor, to ensure the task is performed safely. Finally, you may have to provide PPE (PPE control), as a last resort, to ensure the employee is not exposed to the dust hazard.

That's the beauty of this; you have different controls overlapping one another. Through their interaction and overlap, the exposure risk is reduced and employees are kept healthy and safe.