Linked from
The 14 pages that link to Linear programming duality, each with the reason it gives.
Shadow priceRelated: An optimal dual solution supplies shadow prices for the primal constraints.
Linear programming relaxationRelated: Dual solutions can certify bounds supplied by a relaxation.
Tjalling KoopmansRelated: Dual variables give scarce resources implicit values in allocation models.
Farkas' lemmaRelated: Dual feasible solutions often act as the lemma's infeasibility certificates.