The history of Jagannatha worship dates back beyond any known records.
“Ancient scriptures state that Jagannatha worship has been going on eternally. The Rg Veda mentions it and the Puranas (Skanda, Brahma, Narada) present elaborate descriptions of Sri Ksetra Dhama. The Ramayana and Mahabharata refer to the Deity of Jagannatha Deva.
During the last two thousand years of continuous worship the cult of Jagannatha has assimilated the practices and philosophies of many religious faiths.
Consequently, the cult of Jagannatha worship symbolizes the unity in diversity of all the major religious cults of India, including tree and totem pole worship (the Śabara cult); Jainism; Buddhism; the cults of Purusottama, Vasudeva, Madhava, Rama and Laksmi Nrsimha; and Panca Upasana worship (five-fold Hinduism) as Vaisnavism (Visnu worship), Saivism (Siva worship), Saktism (Durga worship), Ganapatya cult (Ganesh worship) and Saurism (Suryadeva worship).
“Jagannatha represents an integration of all the important Hindu cultures that had flourished in India, including the Vedic, Puranic, Tantric, Smarta, Vaisnava, Jainism, Buddhism and the aboriginal tribes. Jagannatha is worshiped as Visnu or Narayana when He sits on the siṁhāsana in the sanctum sanctorum; as Ganesa during Snana Yatra; as Rudra during the Nava Kalevara ceremony; as Durga in the Sayana festival and as the Sun when riding on the chariot during Ratha-yatra.
Apart from this, pilgrims worship Jagannatha according to their religious persuasion. Ramanuja Vaisnavas worship Jagannatha as Narayana, Nrsimha, Hari; Ramanandi Vaisnavas adore Him as Rama; Gaudiya Vaisnava worship Jagannatha as Kṛṣṇa; and Oriyan Vaisnavas see Jagannatha as the combined form of Rādhā-Kṛṣṇa. Hence, Lord Jagannatha represents an assimilation of all the religious cults, creeds and sectarian philosophies under the broadest sense of the term Hinduism.” (Jagannatha Puri, temple pamphlet.
It appears that people worship Lord Jagannatha according to their own realization.
Sri Caitanya Mahaprabhu and His followers see and worship Sri Jagannatha Deva as identical with Bhagavan Sri Kṛṣṇa, the original, Supreme Personality of Godhead.
The Gaudiya Vaisnava view is supported by the fact that Jagannatha Swami’s pujaris use two kṛṣṇa-mantras in their daily worship of Lord Jagannatha: gopijana vallabhaya svaha and klim kṛṣṇaya govindaya gopijana vallabhaya svaha.
Sri Baladevaji is worshiped as the son of Vasudeva with the mantra om namo bhagavate vasudevaya.
Subhadra is worshiped with the bija-mantra Hrim.
Incidentally, the Oriyan Vaisnavas pronounce Kṛṣṇa as Kṛuṣṇa.”
(‘Jagannatha Puri Guide Book‘ by Mahanidhi Swami)
Learn about Srila Prabhupada’s first Ratha Yatra as a boy in Kolkata (Calcutta).