They are for cutting wire.
http://www.all4food.net/2011/08/30/french-terms-in-vegetable-cutting/
By cutting through its diagonal and cutting off each corner parallel to its diagonal which will leave you with 2 triangles and 2 trapezoids.
Diagonal plier is actually a curious misnaming, as it's main job is cutting wires.
at room temperature
cutting vegetables
Side-cutters or diagonal cutting pliers should be used to cut wires, cables, and other materials where precision cuts are required. They have a cutting edge that is angled to provide a clean cut close to the surface.
diagonal cutting pliers
no, use mineral oil. Vegetable oil can grow bacteria.
at room temp.
Vegetable slicers are a common kitchen gadget used in place of a knife when cutting or slicing vegetables.
Diagonal pliers (alos called wire cutters or diagonal-cutting pliers) are pliers intended for the cutting of wire (they are generally not used to grab or turn anything). They are sometimes called side cutting pliers or side cutters, although these terms are shared by other pliers designs, such as lineman's pliers, and may lead to confusion. The plane defined by the cutting edges of the jaws intersects the joint rivet at an angle or "on a diagonal", hence the name. Instead of using a shearing action as with scissors, they cut by indenting and wedging the wire apart. The jaw edges are ground to a symmetrical "V" shape; thus the two jaws can be visualized to form the letter "X", as seen end-on when fully occluded. The pliers are made of tempered steel and inductive heating and quenching are often used to selectively harden the jaws.