Usually the height is around 14 inches, most benches would be too high for that.
The benefits aren't really dependant on the height of the box, the real benefit is in the correct execution of the lift. The way I see it is this:
1) Using the box to squat as you normally would. The benefit here is just solely to gauge depth.
2) Using the box to actually squat back and down to in correct box squat form. The benefits here are huge and numerous in my opinion.
I've written a little about Box Squats here
and I may expand that to a full article at some point. I may even do a little video on them if there is sufficient interest.