It is for draft control.
Your manual should describe how to use it.
It may help with the box blade, but usually for more ground-engaging attachments like a moldboard plow, or digger.
If they start going too deep, then the draft control senses this load and causes the 3pt arms to raise until the load decreases. A form of position control.
Don't think it will solve the leveling problem.
Draft control. The farther down you have the lever the harder a ground engaging implement will pull before the 3pt arms raise. It most likely wont keep the box drag level accross undulating ground. It is meant to automatically lift the 3pt arms to even out strain on the tractor. In my opinion it is easier to put the draft control on max and control the 3pt itself than get the draft control adjusted right.