Ah right.. mesh splits create physical breaks or holes in the mesh, forcing light/shading to adhere to what such splits define, hard/soft edges. Normal map images over-ride or utilise these splits depending on their being uniquely baked or tiled textures - unique normal maps over-ride because they're high-res versions of the same object, tiled don't because their purpose is to represent 'surface' detailing not an 'object' relative data.
(note: baked normal maps don't actually over-ride, it's more correct to say such mesh objects don't need forced splitting if the object uses uniquely baked normals - if splits exist they tend to adversely affect the appearance of the object because they supersede smoothing based on normals)
A wooden table for example can be mapped with a unique normal map baked from a high-res version of the object. In such instances the normal map defines smoothing/shading (and optionally surface detailing). Mapping the same object with a tiled 'wood' texture however, doesn't work as all that provides is surface detailing, i.e., wood grain. Left in it's raw state this latter approach makes the object appear mushy because the normal map is not providing *object* relative smoothing information (normal orientation as you mention), the mesh is. In such instances it (the mesh) would need to be manually split to define edges and break the uniform surface continuity.
The upshot is that splits tend to be required when models are covered with tiling textures, but not if they are using unique baked data. And yes, the split is caused by a surface break and duplication of vertices along the break - which is what's shown exactly with the pics you've attached... that's the expected behaviour from that arrangement.