Multiple texture coordinates per mesh what is their purpose?
Multiple texture coordinates set can be used to achieve different texture mapping per mesh, this helps achieve different visual effects that are otherwise hard to accomplish and may require multi-pass rendering.
What scenario would these be present?
For example Reflection mapping usually uses different texture coordinate set other than the one used for applying the basic diffuse texture, this is needed to achieve the illusion of reflection, since perfect reflection isn't usually feasible in real time renderings and may not even add much to the rendering quality. Note that texture coordinates that are used for reflection mapping might be calculated at runtime based on normals, in other cases they can be precomuputed.
Other examples for the use of multi-texture coordinates may include light mapping and alpha mapping.