Why do we need and use slang?

What do you know about slang words? Why do we need them? And when do we use them?

Slang words are being used all over the world, especially in native English speaking countries. We can hear slang words in series and movies, although, people, who don't speak English very good, won't understand what they heard. 

People use slang words to make their speech more casual and fast, to connect words. That's why sometimes it's difficult for us to understand native speakers. They speak very fast, connect words without defining each and every word, sometimes they swallow letters and make big words sound very small.

So, please, look at these most common slang words, try to understand why they sound more natural and how they help people to connect words. But notice, slang words are being used in spoken language, don't use it with strangers, during business meeting, etc. Also try not to use them with people who speak very poor English, they won't understand you for sure.