- last post: 01.01.0001 12:00 AM PDT
We don't know what the Halos are for. When Cortana was talking to the Master Chief she was emotional, and was referring to how the Halos would stop the flood. She didn't explicitly state that was what they were designed to do.
The flood spread far too fast for the Forerunners to build the Halos as a defense against them. If it wasn't for the the marines and the Master Chief blowing up the Halo then the Flood would have been off of it aboard stolen covenant cruisers.
No, I think they were built for another purpose, possibly as a general safeguard against any threat like the Flood.