chielo, priestiss of agbala, came to okonkwos compound for what reason

The Oracle's interest in Ezinma is benevolent. Remember that Chielo is a friend of Ekwefi, and the old widow is also particularly fond of Ezinma. Given Ezinma's health troubles, we can infer that the priestess is seeking some kind of spiritual protection for the child. And indeed, in later chapters we learn that Ezinma ceases to be a sickly child after this strange night with the Oracle.