There are internal and external reasons why Krishna came to earth disguised as a devotee of Himself.
The internal reason was that Krishna wanted to taste the happiness of a pure devotee of Krishna, and in particular, He wanted to taste the happiness of the highest pure devotee, Radharani. So He borrowed her mood of love for Him, especially her heightened mood of intense love when He left Vrindavan and she experienced separation from Him.
Externally, He wanted to not just instruct, as He had instructed Arjuna, that one should surrender in love to Him, He wanted to demonstrate it, to give the best possible help to the people of this age. So He started the movement of chanting Hare Krishna in congregation, and He Himself led the way in ecstasy.