I am doing this tutorial of Blender Guru. And about 15 minutes he is UV unwrapping the mesh. Why is he doing that ? Although I am a beginner I think I understand the general purpose of UV unwrapping. Buy not what Blender Guru is trying to establish here.
The cube on the right is flattened/unfolded so a texture can be applied. On the left is what it looks like after it is unfolded. Where it is cut to unfold these are called seams. In the video he places the seams to unfold the object.
By creating a 2D view of the object you can add a texture in an external editor (for example) exactly where you want the texture to be. The 2D version is mapped to the 3D object ... UV Map
It's so he can place the texture on the model. Without unwrapping it first, you can't put a texture on. It's just how it works, you can't project a 2-d picture on a 3-d object unless it is also 2-d.