In the beginning, the primeval Lord manifested as the transcendental Daru Brahman on the shore of the Ocean, and those who go to take shelter in this Supreme Lord are sheltered from all difficulties.
Prior to being placed at the 12th century temple at Puri built by Choda Ganga Deva, Lord Jagannath was most probably worshipped on the seashore.
Lord Jagannath is MahaVishnu and the epic poet Sarala Das (of Pratarudradeva’s time) had described Lord Jagannath as an Avataree and not an Avatar. He is not an incarnation. So, he doesn’t have any life story or leelas or plays, as any other Avatar, like Rama and Krishna.
MahaVishnu is a form of address that signifies the high pedestal on which Lord Jagannath resides, easily the highest form of Vishnu. All avatars (incarnations) of Vishnu spring from Him.