David Sparks wrote about the point of audio messages in iMessages.

Now that we’ve all been using iOS 8 awhile how big of a thing is the ability to send a recording of your voice via iMessages? I’ve done it once for the sole purpose of demonstrating how it works. I thought it may be a generational thing but my kids report they aren’t using it either.

For some languages, it is easier and faster to send audio messages than to have to type the messages by hand. Audio messages would also be useful for people who can speak a language but doesn’t know it well enough to be able to type it out.