Rock geometry
When you go through a loading point, the game creates the next section of the level you're heading into (which partly overlaps the current section), and as part of that, the size and shape of each rock is determined. But that rock geometry isn't fixed in advance; it seems to get randomized to some extent. That can affect how good a rock is at providing cover to yourself or enemies, whether you can climb it to shoot from on top, how easily you can bounce a grenade off it to land among unseen enemies approaching around the corner, how easily you can squeeze a Warthog past it to get through a gap, and so on. When engineering a save for some battle or other, you may therefore be interested in getting some particular rock geometry. Here are some methods you can use, though the first two can be unsuitable for some situations. In most cases I'd be using a restart method.
(1) Travel back method
If you don't like the rock geometry you've got, travel back to the previous section of the level (going through a loading point) then turn around and come through the loading point again so the geometry gets randomized afresh. Repeat until satisfied.
(2) Simple restart method
Save somewhere before the loading point (ideally the preceding checkpoint), or possibly even after the loading point if you're not going to mind doubling back. Head through the loading point (after having doubled back if needed) and play on until able to check the rock geometry of interest. If it's unacceptable, eject the disc and reload to restart from your original save point - or just revert if you haven't had another checkpoint - and try again (repeat until satisfied).
(3) Copy restart method
This method uses a save held on a memory card. The save point would normally be somewhere before the loading point (probably the checkpoint just before it), but it could be after the loading point if you're not going to mind doubling back. Copy the save to your Xbox and start playing it. Head through the loading point (after having doubled back if needed), and save at the next checkpoint or possibly a later one. Now scout ahead to check the rock geometry you're interested in. There's no need to waste time with careful proper gameplay in this recon phase - just get a look at what you want (which could be a long way ahead). If it's acceptable, you know your saved checkpoint will be a good starting point for doing the rest of your set-up work properly. You can get back to it by ejecting the disc and reloading, or just by reverting if you haven't had another checkpoint. Otherwise try the procedure again, restarting with a new copy of the memory card save. Repeat until satisfied.
This method can be far more efficient than the simple restart method because you're not having to bother with proper gameplay in the recon phase; you can just speed through for a look. That's quite an important consideration if the rocks of interest are a long way ahead of the loading point. In addition, the simple restart method can be unusable if you want to produce a base save with acceptable rock geometry, because by the time you reach the rock geometry of interest, you may have had a checkpoint which is too far advanced for giving a suitable base save.
Odd but useful
Possibly the randomization is some sort of efficiency measure to do with minimizing the amount of information that has to be permanently stored to describe a level. It's kind of odd that there isn't just one fixed version of the Halo landscape, but I'm rather glad about it because it increases the amount of fun available, notably in the megabattles. A particular battle can play different ways according to the rock geometry. Also, if Bungie had fixed the geometry in advance, certain tactics could have been permanently impossible. For example, one of my favourite tactics in the rockslide megabattle is to climb up on the large 'central rock' on the plateau and defend from there with a pistol, as described in Death from on high. If Bungie had made that rock fixed and unclimbable, I'd never have had that pleasure. Horror!