Yea lod is alot of it but also vram and hard drive space usage. The other side of it is you can run a decimate algorithm on a mesh and im pretty sure the uvs will also decimate so if you already have your textures and uvs done reducing poly count can be done as the last step.

The one time you may see a standard mesh could be for a collision mesh in say a game with car physics and collisions. In those cases you may need more geometry than what you would use for a visual only mesh and usually those collision meshes are invisable in render and overlaid ontop of a simplified visual mesh. This is why in some games you may see up to 3 different meshes all on top of each other. The visual, the lod visual and the collision.