Michael Jensen has a good post on the Marcan parables (good exegetical observations for a Systematics guy!). He writes:
The parables are important to Jesus’ teaching in Mark because they goad the reader to listen and understand. They proclaim a division in Jesus’ listeners between those inside and those outside. In them the judgement of national Israel foretold in Isaiah is completed. Mark does not leave his readers as perplexed as the disciples however, or as critics such as Kermode and Crossan would like to presume. He enables them to read the parables in the light of what happened to Jesus on the cross and in the tomb.
I would add that the parables largely function in Mark in a very polemical fashion. A.T. Cadoux and J. Jeremias believed that the parables were weapons of controversy. That is validated if we remember that all of the parables in Mark (except one I think) occur in the context of confrontation and challenge to existing paradigms and to the Judean authorities. I think Markus Bockmuehl is right, however, to say that of the parables generally they are about God’s word and God’s kingdom.