yadā hy adharmeṇa tamo-dhiyo nṛpā
jīvanti tatraiṣa hi sattvataḥ kila
dhatte bhagaṁ satyam ṛtaṁ dayāṁ yaśo
bhavāya rūpāṇi dadhad yuge yuge

TRANSLATION

Whenever there are kings and administrators living like animals in the lowest modes of existence, the Lord in His transcendental form manifests His supreme power, the Truth Positive, shows special mercy to the faithful, performs wonderful activities and manifests various transcendental forms as is necessary in different periods and ages.


PURPORT

“As mentioned above, the cosmic creation is the property of the Supreme Lord. This is the basic philosophy of Īśopaniṣad: everything is the property of the Supreme Being. No one should encroach upon the property of the Supreme Lord. One should accept only what is kindly awarded by Him. Therefore, the earth or any other planet or universe is the absolute property of the Lord. . . .

But at times, due to the influence of the ignorance mode of material nature (tamo-guṇa), the lowest of the material modes, kings and administrators come into power without knowledge and responsibility, and such foolish administrators live like animals for the sake of their own personal interest. The result is that the whole atmosphere becomes surcharged with anarchy and vicious elements. Nepotism, bribery, cheating, aggression and, therefore, famine, epidemic, war and similar other disturbing features become prominent in human society.

And the devotees of the Lord or the faithful are persecuted by all means. All these symptoms indicate the time of an incarnation of the Lord to reestablish the principles of religion and to vanquish the maladministrators.

This is also confirmed in the Bhagavad-gītā.

The Lord then appears in His transcendental form without any tinge of material qualities. He descends just to keep the state of His creation in a normal condition.

The normal condition is that the Lord has provided each and every planet with all the needs of the native living beings. They can happily live and execute their predestined occupations to attain salvation at the end, following the rules and regulations mentioned in the revealed scriptures. The material world is created to satisfy the whims of the nitya-baddha, or everlasting conditioned souls, just as naughty boys are provided with playing cradles. Otherwise, there was no need of the material world.

But when they become intoxicated with the power of material science to exploit the resources unlawfully without the sanction of the Lord, and that also only for sense gratification, there is necessity of the Lord's incarnation to chastise the rebellious and to protect the faithful.”

(Srimad-Bhagavatam 1.10.25;
Translation & Purport by AC Bhaktivedanta Swami Prabhupada)