Why do they invoke...

[ProjectIRR]

In example 1 the writer invokes a IAnimatedMeshSceneNode

IAnimatedMesh* mesh = smgr->getMesh("../../media/test.obj");
if (!mesh)
{
device->drop();
return 1;
}
IAnimatedMeshSceneNode* node = smgr->addAnimatedMeshSceneNode( mesh );

In example two the user makes a scene node, then seems to, redefine? it as some kind of other scene node?

scene::IAnimatedMesh* mesh = smgr->getMesh("20kdm2.bsp");
scene::IAnimatedMesh* buildingmesh = smgr->getMesh("../../media/buildingtest2.obj");
scene::ISceneNode* node = 0;
scene::ISceneNode* buildingnode = 0;

if (mesh)
node = smgr->addOctTreeSceneNode(mesh->getMesh(0), 0, -1, 1024);
if (buildingmesh)
buildingnode = smgr->addMeshSceneNode(buildingmesh->getMesh(0));

I added a mesh in the code above

I'm just wondering why he took this seemingly extra step.

[BlindSide]

They are just checking to make sure that the mesh pointer is valid before initializing the scene node. That being said the current examples don't necessarily portray the recommended coding style as they are more designed to show Irrlicht usage in a simple manner rather than the basics of C++ coding.

Cheers