Because some believe it's overused and there could be plenty of separate scenarios that went towards the same ends instead of not even attempting otherwise.
Vague, no doubt, but I call Daggerfall a true ES game and it gave you one character trait "Friend of the King". You can imagine exactly how the king acts, how you are friends with him, why he trusts you with the Daggerfall quest, and so on. One little trait can open up a lot of options if it just defines one little bit of your history. What really gets me is that people are so concerned with where their characters come from, but never care how they progress. They're fine with being the Nevararine and the Dovakiin, but tell them they've got to have one single thing before all this and they flip.