Through lots of experience, I find that Players/PCs are more interested in and attached to that little girl they rescued from the goblins during an actual game session than they are to any little girl they are supposedly related to through their pre-game back story.
Sure, because at the table, it "happened".
The little girl in the backstory, it didn't "happen", someone just wrote about it happening. It may be important, but maybe not.
But if the DM does something with it, at the table, then it has a good chance to become important just like the little girl rescued from the goblins. If the DM shines his spotlight on it and weaves it into the ongoing story, then they have another way in which their players are anchored into his game.