The VSI is brilliant because it is the only instrument which requires no mental processing to interpret it. Needle pointing up - you're going up. Needle pointing down - you're going down.
That's very important when you are experiencing confusion in the cockpit, however that is brought about (the leans, high workload, somatogravic or black hole illusions etc)
The AH needs you to interpret pitch attitude to the background picture and relate that to speed/adjustment, the ASI needs you to read the speed and relate that to the performance, the altimeter needle goes down on the right and up on the left of the instrument etc etc.