The purpose of the Halo Installations is meant as a last resort weapon against the Flood. They kill all biotic organisms within a radius of 25,000 Lightyears, so that all "food" to the Flood would die and therefor the Flood would die from starvation. Installation 00 (The Ark), can remotely detonate all rings at once. There are currently 7 active Rings. I do not know much about the Monitors, but I do know 343 Guilty Spark managed Installation 04 until it's destruction by the UNSC and that he once knew Bornstellar, the Iso-Didact.