Few thoughts on the various Halo climate zones. All speculation.
We know the Librarian was tasked with the cataloging and collecting of all galactic lifeforms. Of course, this process may have only entailed DNA sampling, but there might have also been a plan to "reseed" and subsequently repopulate the galaxy post-cataclysm. Sample environments could be used not only for study, but also for reintroduction of each species. Think zoologically. The Forerunner believe that they carry the burden of salvaging life in the galaxy. That is the mantle they carry.
But that can't be the whole story, because oddly enough, each Halo (at least the one's we've seen) appear to mimic Earth's climate specifically. We do know that the Librarian noted that humans were a special species, and of course, of all surviving sentient beings, humans are tasked as Reclaimers. It could very well be that the Forerunner were simple accommodating the future stewards of the Halo Array, and subsequently, the galaxy. Rolling out the verdant green carpet if you will. Making each ring feel like home.
And then there's the Flood. Until a bunch of overzealous icon hunters come waltzing through, ignoring Forerunner security checks, this nasty species is kept in isolation for scientific purposes. But speaking of "isolation," the facility that bears the name in the proper noun form is hardly a vat of formaldehyde, a cryogenic chamber, or even a simple sealed container. It's an open air display with grass and rock and caverns. It's exposed to the atmosphere. Again, we may think zoologically. To study a species, you dissect, yes, but you also place it in simulated environments to see how it reacts.
The final option is that it just looks cool.