In addition to having a stellar physics model, the objects and terrain in Halo can be dynamically changed during gameplay in a number of ways. In Halo, if there's a huge explosion, or an orbital strike on a piece of flat ground, it will leave a hole that is a permanent part of the landscape. The rocks and dust are particle systems, like the weather in Myth. Nature is full of particle systems, and if you want to create a natural environment, you gotta have particle systems all over the place. If bodies or debris are disappearing, then there should be a plausible explanation.
Nobody likes to watch a carcass just sink into the ground inexplicably.
Well, okay, not all that many people like watching carcasses period. But you get my point. This applies to both the single-player and multiplayer gaming experience. Say that you destroy a command and control center that disrupts communications across the Halo. You may see the effects of such an attack at a later point in the game.