On my box blade, the rear blade pivots.
1) I can get a full cut with the front blade without the rear blade "lifting" it, as others have said.
2) when you use the box blade to "push" material, the blade gets locked against the box, preventing it from just riding up onto the material.
3) if you angle / tilt the box blade way back (using the top link making it longer), you use the rear blade to smooth the material which you are spreading. The blade will smooth any ridges or tracks. Essentially floating the blade, allowing it to smooth any high spots.
After spreading 16 tons of gravel yesterday using the loader and box blade, the final touch was running the "floating blade" back and forth over the area that was just done. It looked like it was hand raked, not a ridge, line, tire mark, or high spot noticed.
On my driveway/ parking area (60' x 300'), 100 tons, after spreading, I literally drove around in circles starting at the outside and working in towards the center. The rear floating blade made it smooth as can be.
I wouldn't own a box blade WITHOUT a pivoting blade! It just takes it to the next level.
I forgot to mention that I have a hydraulic top link..... that makes angling / tilting the box blade backwards super easy!