Tannins are excellent for beef pairing because they create such high acidity. Acidity is what keeps your wine fresh and allows it to cut through layers of fat. Tannin compounds will actually physically break down the fat in your mouth and allow strong meat flavors to be released. As you eat a piece of meat, the fat can coat your mouth and actually stop you from tasting the meat flavor as you continue to eat. Tannins will stop that coating in its tracks and refresh your palate.