The Mesh UV projection uses the mesh's UV coordinates to map the texture to the surface. This is the default behavior for all textures, so in many cases, it's unnecessary to use a Projection node when mapping a texture based on UVs.
This projection applies a spherical mapping for Environment textures and IESAn IES light is the lighting information representing the real-world lighting values for specific light fixtures. For more information, visit http://www.ies.org/lighting/. light distributions. For more control over the projection (mainly rotation), use the Spherical projection in these cases.