Cognitive ergonomics subjects include human-computer interaction, work stress, mental workload, decision-making, reliability and training requirements.
This all comes in handy when designing or evaluating the efficiency of a design or system. You can understand if the information you want to convey is going to be picked up correctly by the user. You will can also use it to layout information and controls in a way that is easier to understand and operate. Finally you will know the limits for how much and what type of information can be absorbed and understood in the given time and setting.