When paper is printed, it tends to curl because more heat is applied to one side than the other. If you try to print on the reverse side, it may jam. A higher quality paper such as Navigator will resist curling and can therefore be used for duplex printing (the more toner or inkjet coverage there is, the higher weight the Navigator paper should be). Good paper opacity is also very important for double-sided printing.

As Navigator paper has high opacity, the print will be clearly legible on both sides, not shaded by the opposite side.

Printing on both sides gives a better look to reports and presentations and reduces the number of sheets used; even if you use Navigator Color Documents 120g.m-2, you are only using the equivalent of 60g paper printed on one side only. 

This is both economically and environmentally friendly.