Hard to explain if you haven't experienced it yourself (?), but:
(1) it dissolves the illusion of separateness, making you acutely aware of the interconnectedness of everything around you, so you grasp (among other things) the impact of your actions on others
(2) it confronts you with your inner landscape (in a way that can greatly assist with resolving fear, grief, and childhood patterns of relating) and highlights your latent strengths, as well as "weaknesses"
(3) in tribal cultures that embrace this rite of passage, adulthood isn't something you declare - it's something an elder recognises in you. Having their guidance for the ceremony would help you interpret what arises, rather than get lost in it, and interpret the insights
(4) a clearly defined threshold, marked by such a ceremony, would underline the serious responsibility that comes with adulthood (moving from "I am the centre" to "I am part of a larger web") and reinforce values to live by
(5) it would address the existential anxiety and spiritual emptiness that are at the root of so many problems in our society
Not sure if I've explained it well, but all that sort of thing!!! 😜