In Black Panther, there was a ceremony where...
the new king was buried
What was the purpose of the ceremony?
The purpose of the ceremonial burial is to travel to the "Ancestral Plane" so that the new king may consult with his ancestors.
In both cases, Erik Killmonger (Michael B. Jordan) and T'Challa, the Black Panther (Chadwick Boseman), meet and speak with their fathers, N'Jobu (Sterling K. Brown) and King T'Chaka (John Kani). In Black Panther's travel, his father transforms from a black panther, and there are other ancestors present.The presence of the black panthers harkens to the origin story of Wakanda at the beginning of the film.
A third burial travel occurs when Black Panther is buried in snow. During that vision, he confronts his father about the murder of his uncle, Killmonger's father.
The act of burial is purely ceremonial and symbolic, and is not needed to travel to or have visions of the Ancestral Plane.
In Black Panther: Wakanda Forever, Shuri's consciousness travels to the Ancestral Plane upon ingestion of the heart-shaped herb despite her not being buried. Relevant dialogue:
Nakia: Shouldn’t we be burning incense or something?
Nakia: Are you sure you don't want me to bury you?
Shuri: No. I need you to have access to my chest. Just in case I go through cardiac arrest.
The ceremonial burial symbolizes the Black Panther's journey to the Ancestral Plane, which is just one of the many planes of the afterlife. The Ancestral Plane-afterlife connection is expounded upon in this post: In Black Panther, what is the Ancestral Plane?.
It can be safely concluded that you need to have visions before you can become king. And for that you need to be buried.
I have a theory. Somehow, the ancestral plane, soul stone and that herb are somehow connected.And seeing how T'Challa's mother is afraid that he is not covered properly (completely in snow), I feel that taking that herb has side effect that can be avoided by covering in some material (sand or snow).