Why might someone sending a text message using an emoticon?

To make sure the message conveys the intended tone

______________

if you say something face-to-face, the other person can see your face and hear the tone of your voice. The flat Words that you say can be misinterpreted, but your tone and expression can provide clarity. If I say, "Yes, I totally believe that.", with a smirk on my face and in an exaggerated tone, you can understand that I'm being sarcastic, and that the actual meaning is the OPPOSITE of what I say.