With the advent of text and email, an interesting phenomenon has sprung up in our language: emoticons.
I rarely see them in formal communications addressed to large audiences, because the larger an audience, the less personal and more general a message becomes. The more formal a message, the more polished it usually is, and the less it will presume upon shared history and personal knowledge. In such communications, nobody needs to stick a smiley or winky face on the end of a joke to say, "This is a joke." That's not the purpose that emoticons serve: they're not intended to emphasize or telegraph what is already clear from a message's content.
Emoticons occur more in short-form and personal communications. They especially occur in messages that used to be transmitted via the spoken word. And what they are doing is taking the place of visual and audio cues that normally accompany spoken words.
Consider the phrase: "Get out of here." In itself, it could be a statement either of dismissal or jest. In face-to-face encounters, we tell the difference partly from context and partly from cues such as tone and loudness of voice, facial expression, and accompanying gestures. A hushed, tense, "Get out of here," accompanied by the turn of a back, is much different from a jovial, "Get out of here!" accompanied by a grin and a playful nudge.
But onscreen, we don't have tone of voice, facial expression, or gestures to help us transmit and decode such messages. And so, rather than clarifying by saying, "I mean this ironically," or, "You have really angered me," we do what we can with punctuation, capital letters, and emoticons to convey the meaning.
Consider this message:
A. I don't like him.
B. I DON'T LIKE HIM!!!
C. I don't like him. ;-)
D. I don't like him. :-P
E. I don't like him. :-(
Version A is plain and unadorned, and is likely to be taken at face value. Imagine the dismay of someone intending that to be coy or ironic if it were taken seriously. B is emphatic, and may express anger, frustration, or even protesting-too-much. C is coy, implying an attraction the writer wants to admit without admitting. D implies disgust, while E suggests antipathy or possibly hurt.
We've probably all seen situations where someone unintentionally offended another by using words online that were meant to be taken one way but were taken another, just as we've seen real-life situations in which words that could sting if taken seriously were softened by a smile or a light tone of voice. Just kidding, the tone and expression say in person, and "LOL" or "jk" or ";-)" we type online.
In a face-to-face conversation, we might give "I'm listening" cues by nodding, maintaining eye contact, smiling, laughing, or clucking a tongue sympathetically. Online, we might just transmit an emoticon or even click a "like" button to say, "I've heard you; I'm here."
I'm not suggesting we all load up our writing with emoticons. I'm just suggesting that we understand their function in communication, and why it is they have crept into our world.