It can be a way of adding intonation and other affective, out-of-band communication back into text. I never really see people use them purely to replace words one-for-one. But interspersed throughout a message, emojis can, as you say, add irony, but also communicate that a message that could be taken as ironic isn't, or communicate some other subtext.
Text is a flattening of speech, and emojis can add some of those missing dimensions back -- and, like our IRL verbal cues, tics, and gestures, they can be hard to decode if you're not "in" on the game.
Text is a flattening of speech, and emojis can add some of those missing dimensions back -- and, like our IRL verbal cues, tics, and gestures, they can be hard to decode if you're not "in" on the game.