For songs by a different artist than their parent album, for example compilations from "Various Artists" or contributions by a guest on a single track of an album, sorting Rhythmbox's music by Artist (to group all of an artist's albums together) will cause the Various Artists songs (or contributions) to be sorted separately.

Consider a movie soundtrack.  If Rhythmbox's music is sorted by Artist, the soundtrack will be scattered among the other albums from the contributors (which is desirable in some cases), but if it could be sorted by Album Artist, the soundtrack would be grouped together as one album, alphabetized with "Various Artists" (which is desirable in other cases).  This sort method is available in most other music players (Windows Media, iTunes, etc.).