SB 4.22.18
sanat-kumara uvaca
sadhu prstam maharaja
sarva-bhuta-hitatmana
bhavata vidusa capi
sadhunam matir idrsi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sanat-kumara said: My dear King Prthu, I am very nicely questioned by you. Such questions are beneficial for all living entities, especially because they are raised by you, who are always thinking of the good of others. Although you know everything, you ask such questions because that is the behavior of saintly persons. Such intelligence is befitting your position.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Prthu was well conversant in transcendental science, yet he presented himself before the Kumaras as one ignorant of it. The idea is that even if a person is very exalted and knows everything, before his superior he should present questions. For instance, although Arjuna knew all the transcendental science, he questioned Krsna as if he did not know. Similarly, Prthu Maharaja knew everything, but he presented himself before the Kumaras as if he did not know anything. The idea is that questions by exalted persons put before the Supreme Personality of Godhead or His devotees are meant for the benefit of the general people. Therefore sometimes great personalities put themselves in that position and inquire from a higher authority because they are always thinking of the benefit of others.