The motivation behind the symbol was to devise a mark to denote sarcasm or anything not to be taken seriously, to prevent misunderstandings.

Given the nature of the community, a good many of the posts were humorous (or attempted humor).  The problem was that if someone made a sarcastic remark, a few readers would fail to get the joke, and each of them would post a lengthy diatribe in  response.  That would stir up more people with more responses, and soon the original thread of the discussion was buried.  In at least one case, a humorous remark was interpreted by someone as a serious safety warning.

The emoticon was an attempt to solve a problem that still seems to persist today, perhaps because people have overused the smileys to the point that most people don’t associate them with their original purpose.

Smiley Lore [Scott E. Fahlman]