It is usually impractical to cook vegetables completely to order. Too much time is required. But if the vegetables have been partially cooked, the time needed to finish them to order is short.
Partially cooking, chilling, and finish-cooking is not as good, nutritionally, as cooking completely to order, but it is almost as good. It’s certainly better than holding vegetables for hours at serving temperature, and it gives the cook complete control over the degree of doneness when served.
