From Toyota:
A rear air suspension with X-REAS Sport Enhancement Suspension is available on Limited models with a V8 engine for improved ride and performance when towing or hauling heavy loads. The air suspension uses linear height sensors to automatically adjust the ride height in accordance with the vehicle load.
My commentary:
The system will only raise if you have a heavy load...the degree to which it is raised is proportional to the load...thus, maintaining the same ride height as before the load (it keeps the 4runner level).
This is needed especially when towing. Because your 4runner is level, braking force is now evenly distributed. If your 4runner did not have this feature, the rear end will sag down, while the front will lift up. During braking, most of the braking power will be at the rear, not the front (due to sagging). With this autoleveling system, the braking will be distributed evenly (front will have more braking power). Of course, you will also have more steering control as your front wheels are more "grounded."
Lastly, the autoleveling feature just makes your 4runner look better when towing.
I don't have this feature on my '02 4runner, but i think it's a great system to have...so i will vote anyway!
Hope this helps.