Apocalyptic literature is filled with imagery. What is the purpose of this kind of literature? What happens when predictions don't come true

Apocalyptic literature is filled with imagery. What is the purpose of this kind of literature? What happens when predictions don't come true according to interpretations? 

Also, our culture seems to have drifted away from letter-writing forms, but this makes up much of the New Testament text. Are these letters applicable to us? Were they meant just for their intended recipients? How can we determine this?