1)the fact that a person sometimes has to dig to get their full,deep,heart-reaching meaning tends to turn back those who do to not love god but who have a mere surface interest and therefore do not desire the trush in their hearts.
2)illustration conceal truths from those who would misuse them and who desire to entrap god's servants.
3)because the hearer is left to apply the principles of the illustration to himself,it can carry to him a clear message of warning and rebuke,at the same time disarming him so that he has no ground to retaliante against the speaker