Firstly, 了is a kind of modal particle in Chinese, and it doesn't have any practicle meaning. 了 is usually used in daily conversation, after the verb and adjective, and it's informal. When you use了, it gives people a sense of friendliness and makes others feel amiable. You can use 了 in your daily life when communicating with your families and friends. However, you'd better not use了 in a formal occasion, otherwise it will make others feel that you are not a bit serious. That's my answer, and I hope it helps.