The cosmic manifestation gives the conditioned souls a chance to go back home, back to Godhead, and that is its main purpose. The Lord is so kind that in the absence of such a manifestation He feels something wanting, and thus the creation takes place
SB Canto 3
The Lord, the undisputed proprietor of everything, was the only seer. The cosmic manifestation was not present at that time, and thus He felt imperfect without His plenary and separated parts and parcels. The material energy was dormant, whereas the internal potency was manifested.
The Lord is the supreme seer because only by His glance did the material energy become active for cosmic manifestation. At that time the seer was there, but the external energy, over which the glance of the Lord is cast, was not present. He felt somewhat insufficient, like a husband feeling lonely in the absence of his wife. This is a poetic simile. The Lord wanted to create the cosmic manifestation to give another chance to the conditioned souls who were dormant in forgetfulness. The cosmic manifestation gives the conditioned souls a chance to go back home, back to Godhead, and that is its main purpose. The Lord is so kind that in the absence of such a manifestation He feels something wanting, and thus the creation takes place. Although the creation of the internal potency was manifested, the other potency appeared to be sleeping, and the Lord wanted to awaken her to activity, just as a husband wants to awaken his wife from the sleeping state for enjoyment. It is the compassion of the Lord for the sleeping energy that He wants to see her awaken for enjoyment like the other wives who are awake. The whole process is to enliven the sleeping conditioned souls to the real life of spiritual consciousness so that they may thus become as perfect as the ever-liberated souls in the Vaikuṇṭhalokas. Since the Lord is sac-cid-ānanda-vigraha (Bs. 5.1), He likes every part and parcel of His different potencies to take part in the blissful rasa because participation with the Lord in His eternal rāsa-līlā is the highest living condition, perfect in spiritual bliss and eternal knowledge.