What are these square shrouds over these towbar nuts?

Those are captive nuts, if they are not rusted then the shroud stops them turning.

If they are rusted or seized then the nuts can rotate in the shroud and you will need to hold them with pliers or mole-grips.

Just undo the bolts ftom the other side.

To your second question,

Generally, caged nuts would be used during assembly when the location of the Nut is difficult to work with, and especially if the 2 parts prefer to not stay together, which would lead to nuts being dropped. By attaching these first 2 in cages attached to the part, working the Bolt from the other would be much easier until some stability had been brought to the structure being assembled.