When will you use uv mapping and when will you use material nodes?
UV mapping is for custom textures, meaning textures that you paint in Blender or Photoshop/Gimp/Krita.
Material nodes are for procedural textures that don't require a custom UV layout.
For example, a character is a complex modeling that would require custom textures to be painted. But a simple stone could be textured procedurally with only nodes; no UV layout or texture paint required.