Slang has had both positive and negative effects on society.
One positive effect is that the use of slang helps to informally develop the language. Languages are constantly changing and growing. English itself is a complex mixture of Greek, Latin, German, and French. As society changed through war and other politics, the English language grew and developed. Slang expressions work in the same way; as society changes and develops, new slang expressions are born, like "hang out," and old slang expressions are dropped, like "hep cat."
However, one negative effect of the use of slang is that sometimes members of society become unable to differentiate between when slang should be used and when formal language should be used, allowing slang to infiltrate what should be scholarly, formal language. Recent news reports reveal that students these days are becoming so familiar with using slang, such as "IDK (I don't know), SMH (shaking my head), and BTW (by the way)," in their everyday communications through social media and texting, that slang terms are now making their way into students' papers. U.S. News reporter Ryan Lytle relates foreign language teacher Terry Wood, at St. Mary's Ryken High School in Leonardtown, Maryland, as stating that there has been a "dramatic decline" in students' writing abilities, and Wood attributes the decline to "Tweeting, Facebook, and texting" ("How Slang Affects Students in the Classroom"). There is such a dramatic decline that, as Wood reports, students do not even "capitalize words or use punctuation anymore" ("How Slang").
The reason why such a dramatic decline in communication abilities due to the influence of slang is such a problem is because formal language must be preserved within a society. Society, especially multiple societies, cannot share knowledge without having a shared, formal language. Historically, the language shared between scholars was Latin. All scholars of science, mathematics, philosophy, and theology wrote in Latin, whether they were of German descent, French descent or any other descent, so that their knowledge could be passed down and understood by others. Today, the language most shared by those in the academic world is English; English is the preferred learned language of nearly every country in the world. So, formal English must be preserved in order to further preserve the ability to share knowledge.