Slang: what is it and why do we use it?

As I entered the American world, a new form of language was presented to me: slang.
Yes, as I came into the U.S. I already knew English pretty well, but I was definitely not updated on the slang. Going to high school, I would hear plenty of words I didn't know from my classmates, such as "hip", "fierce", and "low key".
Now, after having spent almost two years here, I can proudly say I am a slang-used as well, or I'm very "hip" to slang. But this made me question why is there such a big slang culture in the United States?

First, we need to know what slang is. Britannica.com defines it as

Slang[:] unconventional words or phrases that express either something new or something old in a new way. It is flippant, irreverent, indecorous; it may be indecent or obscene. Its colourful metaphors are generally directed at respectability, and it is this succinct, sometimes witty, frequently impertinent social criticism that gives slang its characteristic flavour. Slang, then, includes not just words but words used in a special way in a certain social context. The origin of the word slang itself is obscure; it first appeared in print around 1800, applied to the speech of disreputable and criminal classes in London. The term, however, was probably used much earlier.

I believe that the origin is obscure for the simple fact that not everyone uses slang. In fact, I rarely hear people older than 30 use it. It is commonly used by young people, such as high school students and college students, at least from my experience.
Now, why do we use slang? I found a very interesting answer to this question on Quora,  by Franklin Veaux, published author and compulsive writer:

Because language is a tool, and sometimes slang is the best tool for the job.

There might be a lot of reasons why slang is the best tool for the job:

• It communicates more quickly than formal language
• It communicates ideas, concepts, or descriptors that formal language has not yet caught up with (when this happens within an industry or profession, we call it "jargon;" when it happens socially, we call it "slang;" either way it's the same thing)
• Because it offers side channel or subtext use communication that is understood within a particular group but not outside it (an extreme and fascinating example is Cockney rhyming slang)
• Because it communicates nuances of meaning or emotion better than formal language

When slang becomes popular enough, it stops being slang and becomes part of the language. This is how language evolves. Vernacular is the laboratory and the playground of language.

I highly agree with his last words. Slang that is very particular or not often used is usually not very known. I, as a foreign, probably know only the most famous types of slang words, but I bet there's a whole new world to explore if I were to learn more not well-known slang.