What key message is the poet trying to give through his poem ‘Song of the Rain'?

Very subtly, the poet conveys a message through this poem that rainfall should not be taken as just a common place phenomenon of nature. The occurrence is a wonder of nature and a boon to the Earth and mankind. It deserves to be admired and appreciated as something divine. However, to sense its divinity one needs to have refined sensibilities. One should thus recognise its significance and understand how important it is for Earth.
The poet through the poem also reveals how nature makes rain an instrument to enhance its beauty and to decorate it. The rain, in other words, rejuvenates Earth and fills joy in every being.