How is the Big Picture Planned Out?

[Kwalitee]

If the whole point of the halos is to wipe the universe of any civilizations who might be approaching the Forerunner's level of civilization, why create the Ark in the first place?

The Ark has two purposes I can think of:

-If its purpose is to save Earth, why aren't they concerned with an advanced society evolving on Earth?
-If its purpose is to remotely activate the halos, why put that facility on a planet? Why not just have each halo signal the others? Or why not build it in orbit like the others?

In addition, Guilty Spark stated specifically that the halos were designed to wipe the galaxy of life, "or at least any life with sufficient biomass to sustain the flood." What if a sapient species developed that had a biological structure smaller than required by the Flood?

[Kiloh Ekim]

If the whole point of the halos is to wipe the universe of any civilizations who might be approaching the Forerunner's level of civilization, why create the Ark in the first place?

Genetic control. If you leave everything in the evolution pool to chance what will you be left with after everything's been said and done? Random abominations (I'm looking at you Covenant). The forerunners created and left the first humans on the Ark to carry on their legacy. Possibly genetically altering us along the way to make us naturally curious and reckless. They were playing (or being) God. I'm also thinking they made us in their image but that's only a assumption.

The Ark has two purposes I can think of:

-If its purpose is to save Earth, why aren't they concerned with an advanced society evolving on Earth?

It depends on how the Ark stops the halos. Either it's a force field on the outside of the ark or a generator to cover the entire planet. My guess is with the former (for storytelling reasons, it would be better. And that's what it's going to come down to.)

There wouldn't be an advanced society left either way. Even if the flood couldn't breach the Ark, they would still destroy everything outside. They would annihilate all of Earth's cities and technology. You would be left with what, the knowledge to create fire? You would have to start from scratch one way or another.

-If its purpose is to remotely activate the halos, why put that facility on a planet? Why not just have each halo signal the others? Or why not build it in orbit like the others?

I feel the need to quote from Shawn of the Dead: "We need someplace more secure, somewhere on the ground."
It's logically the safest place to put a station like that. If you can survive to board the Ark and activate the halos, the flood are still outside, destroying your vassal. If you're in space, this holds quite a predicament. On land, when it's all over, you can start a civilization right there, no ship or technology required.

[Frogwart]

I feel the need to quote from Shawn of the Dead: "We need someplace more secure, somewhere on the ground."
It's logically the safest place to put a station like that. If you can survive to board the Ark and activate the halos, the flood are still outside, destroying your vessel. If you're in space, this holds quite a predicament. On land, when it's all over, you can start a civilization right there, no ship or technology required.

Maybe the Forerunner built the Halo installations and their home base in the African plains. When they learned of the Flood from one of their colonies on the Halos, they activated the rings from Earth. The few left to survive here started society as we know it by joining the people already here on Earth (N'Chala and others), not knowing their colonists out on the installations had kept some Flood samples for future study...

[Kwalitee]

I agree when you say they probably made humans in their own image. Even if we aren't the Forerunner per se, we must resemble them; otherwise, Guilty Spark et al wouldn't have called us Reclaimers.

But I suppose my issue is that it seems like the Forerunner assumed that humans wouldn't cause problems in the future. I understand that human civilization would be completely back to square one...but we'd still be alive and capable of advancement without them around to trigger the halos. Did they trust us to carry on their legacy, to understand what we were supposed to do/not do?

As it stands, it doesn't really look like there was much of a civilization on Earth at the time of the first firing anyway (unless the comic just showed a native tribe out of context of the surrounding nations), so wiping a slate clean that was hardly dirty seems unnecessary. Of course, maybe that's just a side-effect of absolutely having to fire the halos just then, but eh.

Hmm...unless they figured that we'd never move much further up than huts and spears. But either way, I see it as the Forerunner having an extreme issue with hubris. Of course, they did invent the Flood and so on, so maybe. I dunno. It's midnight and this is all pretty much inner monologue.