Why do we use slang? (Biology/Philosophy)

I believe that slang is a subliminal interpretive excuse, which everyone uses from time to time.  The reasons people start saying slang are different for different people, and it's not something that can be courteously generalized.  English, as well as any language, cannot completely convey what is on the human mind.  Even if so, time would pass, rendering it disturbing.  There are 2 ways we in America (from my recollection) know of the term slang: 1) in a cussing verbal expression 2) as a pseudonym to another word or similar meaning.

People and Slang:

Slang 1) "darn", "gawd", "ow"  These are rough translations of what could come out of our mouths naturally, for various obvious reasons.

Slang 2) a) Can be derived from a singular use, as jargon (Jawn or Ass), where it methodically propagates throughout society.

b) can be derived immediately by not being able to come up with a suitable word.  The word could fit into an impossible sentence that someone wants to state appropriately.

c) could be culturally related, where several people do not identify with the local lingo; and therefore devise replacement lingo. [English language] This is the most common form of slang.