It is probably one of the hardest aspects of the English language to learn, because there are so many variations in each small town, let alone in different English-speaking countries!
People use slang to sound less formal. Slang is mainly used because it relaxes the mouth, and allows people to speak in a more comfortable way. It is a lot easier to use slang than ‘perfect English’, which is probably why it’s so common!
Data taken from: http://www.espressoenglish.net/slang-words-food-and-drink/