The lack of body language, facial expressions, and tones of voice in "chat", can cause misunderstandings.
Emoticons (as well as abbreviations and acronyms) help clarify and express emotions.
Consider this phrase in different tones of voice: "That was really nice."
That was really nice :-) <said with a smile>
That was really nice :-( <said with a frown>
That was really nice :-Þ <sticking out your tongue>
Totally different meanings, clarified with an emoticon..
Creative people come up with new emoticons constantly, here are some of the basic ones...