Out of his mercy, because he is a Vaisnava, a devotee, Arjuna is opening the understanding for the common man as far as the all-pervasiveness of the Supreme is concerned. BG 1972 purports

Expressions researched:
"Out of his mercy, because he is a Vaiṣṇava, a devotee, Arjuna is opening the understanding for the common man as far as the all-pervasiveness of the Supreme is concerned"

Bhagavad-gita As it is

BG Chapters 7 - 12

Now Arjuna is convinced that His friend, Kṛṣṇa, is the Supreme Godhead, but he wants to know the general process by which the all-pervading Lord can be understood by the common man. No common man, including the demons and atheists, can know Kṛṣṇa because He is guarded by His yoga-māyā energy. Again, these questions are asked by Arjuna for their benefit. The superior devotee is not only concerned for his own understanding, but for the understanding of all mankind. Out of his mercy, because he is a Vaiṣṇava, a devotee, Arjuna is opening the understanding for the common man as far as the all-pervasiveness of the Supreme is concerned.

BG 10.17 (1972), Translation and Purport

O Kṛṣṇa, O supreme mystic, how shall I constantly think of You, and how shall I know You? In what various forms are You to be remembered, O Supreme Personality of Godhead?

As it is stated in the previous chapter, the Supreme Personality of Godhead is covered by His yoga-māyā. Only surrendered souls and devotees can see Him. Now Arjuna is convinced that His friend, Kṛṣṇa, is the Supreme Godhead, but he wants to know the general process by which the all-pervading Lord can be understood by the common man. No common man, including the demons and atheists, can know Kṛṣṇa because He is guarded by His yoga-māyā energy. Again, these questions are asked by Arjuna for their benefit. The superior devotee is not only concerned for his own understanding, but for the understanding of all mankind. Out of his mercy, because he is a Vaiṣṇava, a devotee, Arjuna is opening the understanding for the common man as far as the all-pervasiveness of the Supreme is concerned. He addresses Kṛṣṇa specifically as yogin because Śrī Kṛṣṇa is the master of the yoga-māyā energy by which He is covered and uncovered to the common man. The common man who has no love for Kṛṣṇa cannot always think of Kṛṣṇa; therefore he has to think materially. Arjuna is considering the mode of thinking of the materialistic persons of this world. Because materialists cannot understand Kṛṣṇa spiritually, they are advised to concentrate the mind on physical things and try to see how Kṛṣṇa is manifested by physical representations.