I can't understand why and when should I use Recalculate operator in Shading Options?
Normals are the lines that indicate the direction of the vertices. Sometimes the forward facing verts might be facing backward and your mesh looks darker from outside but lighter from the inside. Sometimes some faces might have correct normals while others might have incorrect normals. So you need to recalculate the normals to fix these shading issues. You generally do this when your mesh looks darker from outside and lighter in the inside, or when you have any shading isssues. To see exactly what i mean. Just add a sphere and scale it with some negative value (like -1). You will see the mesh will be darker because the inner faces which were darker are now outside and outer faces which was lighter is now inside. Then just recalculate the normals in edit mode and it will look fine.