Animals were offered in sacrifice in order to give them renewed life; that was the purpose of having animals there. Offering an animal in sacrifice and giving him renewed life was the evidence of the strength of chanting mantras. Unfortunately, when Dakṣa's sacrifice was devastated by Lord Śiva, some of the animals were killed. (One was killed just to replace the head of Dakṣa.) Their bodies were lying about, and the sacrificial arena was turned into a crematorium. Thus the real purpose of yajña was lost.
Lord Viṣṇu, being the ultimate objective of such sacrificial ceremonies, was requested by the wives of the priests to glance over the yajña arena with His causeless mercy so that the routine work of the yajña might be continued. The purport here is that animals should not be unnecessarily killed. They were used to prove the strength of the mantras and were to have been rejuvenated by the use of the mantras. They should not have been killed, as they were by Lord Śiva to replace the head of Dakṣa with an animal's head. It was pleasing to see an animal sacrificed and rejuvenated, and that pleasing atmosphere had been lost. The wives of the priests requested that the animals be brought back to life by the glance of Lord Viṣṇu to make a pleasing yajña.