Lord Chaitanya (also known as Sri Krishna Chaitanya or Sri Chaitanya Mahaprabhu) is Lord Krishna Himself in His most merciful form - the form of a devotee. Why would Krishna, the Supreme Lord, come as a devotee? One reason is to show the world by His personal example what devotion to Krishna looks like. This was necessary because thousands of years had passed since He had spoken the Bhagavad-gita, and society had become so degraded that the science of bhakti, or devotional service, was elusive to even the greatest religious scholars of the time.
Aware of the mentality of people in the modern age, Krishna considered: There is no use in just telling people what to do. I need to go and show them. Therefore, Krishna came again as Lord Chaitanya to revive the process of devotional service, not as a teacher but as a practitioner.
Because Lord Chaitanya did not present Himself as God, most people were unaware of His divinity. His position was like that of a good teacher who, seeing a child struggling to write the letters A,B,C, takes the child’s pencil in her own hand and shows the child how to make the letters. Although the teacher is sitting at the desk writing A,B,C, it doesn’t mean that she has become a student. She is still the teacher. In the same way, Lord Chaitanya, though demonstrating how to be a devotee of Krishna, remains the Supreme Lord Krishna Himself with all divine potencies.
Another reason that Krishna comes as a devotee is to experience the joy of devotional service to Himself. Devotional service places one in a direct personal relationship with Krishna wherein once accesses a level of spiritual pleasure that derides the happiness of liberation. Although Krishna is omniscient and omnipotent, one thing that He cannot know is what it is like to be in a relationship with Him. As long as He remains in His position as the Supreme Being, He can simply marvel at His devotes’ love for Him and the unfathomable ecstatic symptoms they experience.