Rain is always welcome after a long, hot, dry, Australian summer. The grass goes green, things flower, and the place generally looks better and more productive.
The desert-dwelling Israelites would similarly have understood the value of rain.
So Moses’ description of his teaching as rain would have had a practical meaning. It would have been more than just a song.
Deuteronomy 32:2Let my teaching drop as the rain,
My speech distill as the dew,
As raindrops on the tender herb,
And as showers on the grass.