Then why do people use slang?
For a number of reasons. To be picturesque, arresting, striking and, above all, different from others. To avoid the tedium of outmoded hackneyed "common" words. To demonstrate one's spiritual independence and daring. To sound "modern" and "up-to-date".
It doesn't mean that all these aims are achieved by using slang. Nor are they put in so many words by those using slang on the conscious level. But these are the main reasons for using slang as explained by modern psychologists and linguists.
The circle of users of slang is more narrow than that of colloquialisms. It is mainly used by the young and uneducated. Yet, slang's colourful and humorous quality makes it catching, so that a considerable part of slang may become accepted by nearly all the groups of speakers.
H. W. Fowler defines a dialect as "a variety of a language which prevails in a district, with local peculiarities of vocabulary, pronunciation and phrase".  England is a small country, yet it has many dialects which have their own distinctive features (e. g. the Lancashire, Dorsetshire, Norfolk dialects).
So dialects are regional forms of English. Standard English is defined by the Random House Dictionary as the English language as it is written and spoken by literate people in both formal and informal usage and that is universally current while incorporating regional differences. 
Dialectal peculiarities, especially those of vocabulary, are constantly being incorporated into everyday colloquial speech or slang. From these levels they can be transferred into the common stock, i. e. words which are not stylistically marked (see "The Basic Vocabulary", Ch. 2) and a few of them even into formal speech