Basses, the lower of the two male voice types in a standard choir, tend to be grouped together in the back rows. Partly this is to do with height (can't have those divas a.k.a. sopranos being hidden!), and partly to do with balance. Those boys can sing loud!
If you've ever wondered what's going on in the back rows, here's an amusing insight by Jim Ramlet a seasoned chorister with VocalEssence Ensemble Singers.
Bass (singer): This refers to basses in choir. Not the musical instrument or the fish.
A bass in [a] choir is a man that sings in the lowest range. The kind of man whose voice seems to literally make the stage vibrate with its low, resonant, reverberating and rich sound quality. The kind of man who kicks baritone and tenor arses collectively. The kind of man who adds depth to any choir song.
I'm a soprano :)