Helpful little paragraph on parables from thr NET Bible note on Matthew 13:2
Though parables can contain a variety of figures of speech (cf. the remainder of chapter 13), many times they are simply stories that attempt to teach spiritual truth (which is unknown to the hearers) by using a comparison with something known to the hearers. In general, parables usually advance a single idea, though there may be many parts and characters in a single parable and subordinate ideas may expand the main idea further. The beauty of using the parable as a teaching device is that it draws the listener into the story, elicits an evaluation, and demands a response.