Pinch yellowing leaves off tomato plants inhibits diseases and makes the plant look better. Removing unnecessary leaves also helps the plant use more of its energy for growing fruit.
A tomato plant needs plenty of air and light to be productive and free of disease. Yellowed leaves block sunlight and the plant has less energy to produce fruit, or the fruit may be small. Water collects on foliage and too many yellow leaves prevents air from circulating. Tomato rot and leaf spot develop when there is too much moisture present. Don't remove too many leaves as heavily pruning a tomato plant results in sun scald.