Optimization is always a good thing. After all, we're tinkering with an old game, and who knows how it will react if we overload it.
LOD (Level of Detail) is a good thing that is still used and will be used in modern games. It's a shame that it can't be properly configured in Heroes 5. But the option of simply reducing the load on the global map is also a good one. I don't think it affects the visual aspect in any way, because in the game, on the global map, you look at the creatures in a very small size and don't really see much.