Emoji serve a similar purpose to gestures.
From a functional perspective - emoji fulfill a very similar role in chat-based conversations as gestures do in real-world in-person conversations.
They carry some semantic meaning, but not much. They, however, can provide great context to the words that are used.
I had fun 😄. is very different than I had fun 😴.
The smiley face :-) was invented as a suggestion to prevent sarcastic messages from being taken seriously.