tes4lodgen scans all your installed mods for buildings and structures and stuff like that to represent in game as LOD. Doesn't handle land meshes.

Works best with RAEVWD.

There are (at least) two different types of LOD, and this causes a bit of confusion. The LOD files coming with the UL mods, are landscape LOD, e.g. the land meshes you see from afar. If you don't install this, the landscape will look like vanilla landscape from afar, and then suddenly change when you come nearer.

The LOD created by TES4LODGen however, is for the distant objects, like houses and ruins. If you don't run TES4LODGen, you will see the terrain, but not see the buildings until you get close enough that the cell becomes loaded - thus a popup effect. TES4LODGen scans your current load order and generates objects according to it - but note that to generate a LOD object, it needs _far meshes for the objects, and most objects don't have those. But this is where RAEVWD comes in - it adds _far meshes for ruins etc., allowing TES4LODGen to generate the LOD objects for them.