When crafting an album, artists tend to look at the collection of songs as a complete experience rather than just a hodgepodge of tunes. Once it's finished, that's when the marketing team starts taking a closer look at the tracks to find which song has the best hook for the audience to grasp. Hence: the single.
Granted, there are many cases where the song released as a single is the best song on the album (i.e. "Hotel California" and "Whole Lotta Love.") However, releasing a single means that you're playing with fire artistically. No matter how great the album is, its identity will more often than not be tied to that one single.
That being said, across some fantastic albums are songs that never got released as singles. While the album tracks tend to fall into obscurity, some of these tracks have had a pretty strong shelf life even if they don't have the same draw as the initial single. If you peel back the layers of an album you can find some tunes that have become not only fan favorites, but some of the greatest songs these bands have ever written.