Linked from
The 4 pages that link to Mirin, each with the reason it gives.
SakeCompared with: Mirin is chiefly used as a cooking seasoning, unlike sake’s role as a beverage.
KojiRelated: Koji enzymes generate sugars during mirin’s rice fermentation.
KatsudonRelated: Its sweetness rounds out the salty sauce commonly used for katsudon.
GyūdonRelated: Its sweetness and aroma help round out the soy-based sauce.