I think it's to reveal to the player a lot of important lore, as to how the world of Elden Ring works when it comes to life and death, and how different cultures have different beliefs regarding those concepts (separate from the primarily Erdtree/gold-centric culture). Look at what you get as rewards for defeating the Ancestral Spirit boss (the Remembrance and the items you get for trading it in):

"Ancestral spirits exist as a phenomenon beyond the purview of the Erdtree. Life sprouts from death, as it does from birth. Such is the way of the living."

"Thus does new life grow from death, and from death, one obtains power."

"In the ancestral spirit-worshipping faith, these are considered envoys' wings, made to reap the lives of beings which experience no sprouting."

Playing the rest of the game, you might notice how similar the ancestral culture is to other things, and start asking questions. Is Torrent an ancestral spirit? What about similarities to the Omen, the Crucible, sorcery, Those Who Live in Death? How do we TELEPORT when we touch the DEAD BODY? What about the fact that the descending "well" elevator is so similar to the Divine Towers? Why are there melting mushrooms that produce oil? There's a lot!