One impotant reason is implied by Celtaddict, I think. You just up and sing a song to your kith or kin without any announcement of the title or background. Three weeks later the kith or kin (or my spouse) says Hey, sing that song again. Huh? you say. You know, the one about the man. Huh? you say --- what man?

Oh, you know, the man with the backpack. So you sing Child #279 (appendix). It now becomes permanently entitled among all who know you as "The Man with the Backpack."