why is breathing important during exercise?

I train karate for three months now, and it's everything I wished for from a martial art. Hard exercises, great fighting techniques, motivation lifting, and of course, regulation of breathing. Why is breathing important during exercise? We've been told many times during classes of its importance, but I need more detailed explanation.

Actually, it's very simple. Let's take this subject from the basics, If we don't breath we die. Therefore, oxygen and its distribution during hard training is very important. Without proper breathing, lactic acid compiles very fast in our muscle cells, thus preventing them to act as fast as we would like. In relaxed fitness exercises maybe this is not that important, you can always take a break, but when you perform precise techniques, maximum flow of oxygen to cells in the right moment is essential. Without that, your blows are slower and less effective.