[jcrawfordor]

This technique is often used to reduce eyestrain and refocusing by extending the optical distance to the display, making it.more similar in terms of vergence angle and focus distance to the view outside. I'm not sure if this was the motivation in the Prius but given the fairly advanced state of VFD displays at the time I'd guess so, there would have been options to avoid the complexity. It's further curious that the Prius used to use half mirrors to combine multiple display units for that dashboard, I'm less sure of the reason for that other than that perhaps the assembly was expensive and they saved money by doing it only for critical instruments like the speedometer.