You may have noticed that your tomato plants have finally gotten the clue and started to grow a little bit. You may have also noticed that, once they start to grow, they can get out of control pretty quickly...thus the need for trellising and...pruning. Believe it or not, but cutting out some of the branches of your plant will actually result in more, better tasting fruit (and your tomatoes will be less prone to problems such as fungus, mold, drunk driving, etc.). Brad found this article which explains it all very well:
Pruning Tomatoes - Fine Gardening Article 
Source: finegardening.com