If part of the mesh is visible outside then the pixels of the lightmap can bleed over onto the interior, so by making it meet in the corners exactly it prevents the 3D meshes from not being aligned with the pixels of the lightmap.
If part of the mesh is visible outside then the pixels of the lightmap can bleed over onto the interior, so by making it meet in the corners exactly it prevents the 3D meshes from not being aligned with the pixels of the lightmap.