1 reason for warming up and cooling down is because if you don't warm up during your dancing session, you could easily pull a muscle or damage something in your body. It is the exact same for cooling...
Because otherwise you risk pulling a muscle, and probably it will save you from feeling all stiff and sore in the morning, hope this helps,Janet WalkerUniversity of Dance, London
If you don't warm up you increase the risk of injury through strained muscles, and if you don't cool down there's a bigger risk of you feeling sore and stiff the next day.