As swarms of moths enter into the blazing fire only to get perished, so are the worlds entering Your mouths with tremendous speed to court destruction.
Rivers flowing from great heights to the sea-depths below, are relevant, cyclic and useful. Seas alone send up the salt-free vapour to hover in the sky, get cooled and shed on great heights. The rain water forming rivulets and streams, flows to the plains below, catering to the needs of humans and non-humans variously. The cyclic process, as many others in nature, continues to sustain terrestrial life.
Is the plight of moths similar? They enter the flames of fire only to get perished. Are the worlds entering the mouth of the Universal form alike? Arjuna is stunned and greatly frightened to think that destruction is the culmination, the final order of life!