Linked from
The 6 pages that link to Roux, each with the reason it gives.
Starch gelatinizationRelated: Its thickening depends on starch gelatinizing after liquid is added.
Cream soupRelated: A roux thickens many cream soups before liquid and dairy are added.
Macaroni and cheeseBroader topic: A roux thickens stovetop cheese sauce and helps it cling to pasta.
CroquetteRelated: A firm roux-based filling helps croquettes keep their shape before frying.
FricasseeRelated: A roux can thicken the white sauce used to braise fricassee.