The original gusseted construction incorporates a "delta" shaped piece of material sewn into the crotch of the jean. This allows stress to be redistributed around the crotch area, allowing greater freedom of movement, comfort and strength.
As an added benefit, the original set-turn-and-topstitch construction is stronger and less thick. The result is 3 ply leg seams, and only 9 ply thick at the crotch (not 16!).
Definition of a Gusset
From Merriam-Webster Dictionary
Date: Circa 1570.
Noun: a usually diamond-shaped or triangular insert in a seam (as of a sleeve, pocketbook, or shoe upper, or jeans* to provide expansion or reinforcement. *(not in the Webster's definition)