If you make vague predictions in an engaging way, your audience will figure out how these vague statements fit within their personal worldview. In cognitive psychology this is known as the Forer effect and helps explain the popularity of horoscopes and personality tests.
Cormac McCarthy's novel The Road is a classic example of this. The book has been adopted by such diverse fan bases as science fiction enthusiasts, environmentalists, and religious people. Although the novel has no explicit passages that refer to any of these groups, it describes a general landscape that each of the groups can rally behind.
It is a strange human phenomenon that the most appealing works of art are often so vague.