It tenderizes connective tissue, if moisture is present and cooking is slow. It toughens protein. Even meats low in connective tissue can be tough if cooked at excessively high heats for too long. The principle of low-heat cooking 1.
High heat toughens and shrinks protein and results in excessive moisture loss.
Therefore, low heat cooking should be the general practice for most meat cooking methods. Broiling seems to be a contradiction to this rule. It takes time for the heat to be conducted to the interior of the meat, so the inside never gets very hot. Meat broiled well done, however, is likely to be dry. Roasts cooked at low temperature have better yields than those roasted at high heat.