Moses' song is likened unto rain and dew primarily because the content of his song can bring life to those who hearken. And what was the content of his song? What was so life-giving and refreshing about it? Verse 3 says: "...I will publish the name of the LORD." That means he will proclaim the name of the LORD. That is why his song is special. It is special because he is declaring to all who God really is. God has several names, but the name that is frequently used in the Bible is that which is revealed to us as Yaweh or LORD (in all caps).
Recognizing who God is and submission to Him will result in having a meaningful and fruitful life. Moses has come to the end of his life, although he was barred from entering into Canaan, he did not allow God's punishment to embitter his spirit. Moses rather declares to the Israelites the greatness and steadfastness of the LORD, and the one responsibility they had, and we do as well, is found in the latter part of verse 3 "...ascribe ye greatness unto our God." This could very well be every living creature's purpose: to ascribe greatness unto our God.