The Lord (Krsna) was acting as the spiritual master of Arjuna. Therefore it was His duty to inquire from Arjuna whether he understood the whole Bhagavad-gita in its proper perspective. BG 1972 purports

Expressions researched:
"The Lord was acting as the spiritual master of Arjuna. Therefore it was His duty to inquire from Arjuna whether he understood the whole Bhagavad-gita in its proper perspective"

BG Chapters 13 - 18

The Lord was acting as the spiritual master of Arjuna. Therefore it was His duty to inquire from Arjuna whether he understood the whole Bhagavad-gītā in its proper perspective. If not, the Lord was ready to re-explain any point, or the whole Bhagavad-gītā if so required. Actually, anyone who hears Bhagavad-gītā from a bona fide spiritual master like Kṛṣṇa or His representative will find that all his ignorance is dispelled.

BG 18.72 (1972), Translation and Purport

O conqueror of wealth, Arjuna, have you heard this attentively with your mind? And are your illusions and ignorance now dispelled?

The Lord was acting as the spiritual master of Arjuna. Therefore it was His duty to inquire from Arjuna whether he understood the whole Bhagavad-gītā in its proper perspective. If not, the Lord was ready to re-explain any point, or the whole Bhagavad-gītā if so required. Actually, anyone who hears Bhagavad-gītā from a bona fide spiritual master like Kṛṣṇa or His representative will find that all his ignorance is dispelled. Bhagavad-gītā is not an ordinary book written by a poet or fiction writer; it is spoken by the Supreme Personality of Godhead. Any person, if he is fortunate enough to hear these teachings from Kṛṣṇa or from His bona fide spiritual representative, is sure to become a liberated person and get out of the darkness of ignorance.