The Mekhilta has established that eating meat cooked in milk is forbidden. But what about deriving other forms of benefit — selling the mixture, using it as animal feed, or extracting value from it in any non-eating capacity?
The proof comes through an a fortiori argument involving arlah — the fruit of trees during their first three years, which is forbidden for any benefit. Arlah fruit involves no human transgression in its creation. The tree simply grew and bore fruit. No one cooked it, processed it, or violated any law by producing it. Yet benefit from arlah is completely forbidden.
Meat cooked in milk, by contrast, involves an active transgression — the cooking itself violates a Torah prohibition. If arlah — where no transgression was committed in its production — is forbidden for all benefit, then meat cooked in milk — where the very act of creation involved a transgression — must certainly be forbidden for all benefit.
This argument closes the final gap in the meat-and-milk prohibition. Cooking: explicitly forbidden. Eating: derived through multiple a fortiori arguments. Benefit: derived through this arlah comparison. The prohibition is now comprehensive. You may not cook meat in milk, eat it, sell it, feed it to animals, or derive any value from it whatsoever. Three separate derivations, building on each other, construct a total prohibition from a single stated rule.