The Album Artist usually reflects what is written on the spine of the physical release, it usually consists of the Composer(s) of the Classical music on the release, and the principal performers and choir, orchestra or other ensembles involved, the Album Artist helps to distinguish one release from another.
But if you additionally use the Album Artist to identify only the main performers you can use this option to remove the Composer from this field, since they are not a actually a performer. Using the Album Artist instead of the Track Artist to identify performers allows you to concentrate on the main performers of the release as a whole without catching a performer that may only have performed on a single track on the release.