The Alshich explains this posuk as a prayer. Moshe asks that the effect of his words shall resemble that produced by rain and dew, in that they lead to the growth of crops. Similarly, may his words have a positive effect in developing the people to bring them closer to Hashem. The Chezkuni explains that the effects of rain are not immediately noticeable. It is only after some time has elapsed and grass begins to sprout, flowers bloom, and fruits grow that its worth is appreciated. Similarly, a man cannot be impatient in the study of Torah. He should not despair when he does not realize initial success in his studies. Through patience, resolve, and continued study not only will he have scholastic success, but ultimately his character and personality will be molded into that of a true ben–Torah.
There is yet another interpretation to this analogy. Rain has the natural power to spur growth only after the necessary soil preparations have been performed. Plowing, seeding, and fertilizing are necessary prerequisites for rain to achieve successful results. Similiarly, Torah study success can only be realized after a person has thoroughly geared himself for it. Unless one invests time and effort to ready himself for its effects, it will be similar to rain that falls on barren unseeded earth; it will be for naught.