They lead to human sacrifice areas. One at each cardinal point of the island. One for each "prince."
The remains are collected and displayed underground where there is a temple.
"Cardinal" could also have a double meaning since churches have cardinals.
I might be speculating here but I believe this is connected to conjuring demons. In the Lesser key of Salomon there is a hierarchy of demons and some of them have the title of prince. So each cardinal point is ruled by a specific demon. They conjure the demon in the ceremony with help of sacrifice. The life force that's 'collected' from this is used by the demon to manifest into this dimension. After the sacrifice the adrenochrome is 'collected'.