Oh, well I was taught that letting anger out was easier ( and safer) than holding it all in. When you hold on emotions, it builds over time. Eventually it will erupt when you least expect it. I was taught by my folks, but I believe this myth came from old folklore. It was a lesson on controlling anger, but not lettimg yourself be pushed around. I believe this became common knowledge in schools. Its easy to teach children to hold it all in, but not really safer. Better to be anger no, than later.
Hope this helps, cheers.