Abbreviations are an efficient way of standardizing jargon so that people don't end up using different phrases to mean the same thing, which can cause more confusion than using an abbreviation ever would. In this way they make discussions more clear in the long run, even though they do require newcomers to familiarize themselves with the argot.

They also, of course, serve as an in-group identifier that helps the audience to recognize how familiar the speaker/writer is with the subject matter, which in turn often subtly affects how seriously their points are taken.