There are 3 main reasons for pruning tomato plants, and they all have to do with producing a more plentiful and healthier tomato harvest. First, tomato leaves and branches that reach down and land on the ground (the ones you often see turning yellow) can be a great host for diseases or bugs just waiting to hitch a ride. By trimming these low-hanging off-shoots you can make it harder for those diseases to spread. Second, plant diseases also spread more readily in a tomato plant that is thick with foliage all clumped together. By pruning your tomato plant, you create air circulation, thus helping the breeze to blow through and making it harder for diseases to spread from leaf to leaf. You also lighten the load a bit, meaning your plant will have less weight and may not droop over as badly. Third, as the end of the season gets closer you will want your plant to focus more on growing fruit than on growing leaves. By starting to trim new growth at the top of the tomato plant, you will signal your plant to stop producing greenery and encourage fruit to ripen. Basically, you're telling the plant it can stop growing taller and start making those tomatoes fatter.