When we are working out we usually are contracting or forcing our muscles. After each workout we are supposed to stretch to reset our muscles and let our body breathe and return to its normal state. If we don’t do this, we may pay with soreness or pain.
I agree completely with what Trinity said. Stretching is actually a very important part after the work out and I’d even consider it necessary so that all our muscles can actually find a way to be released. I’ve found from experience that when we don’t stretch we do suffer from pain and soreness afterwards, more than normally!
Lactic acid is produce in every workout as an anaerobic process of our muscles to obtain energy. This substance stores in the muscle causing soreness. When we stretch we prevent lactic acid to stay in the muscle.