More than help, PC backstories are essential to my campaigns. In fact, I tend to build them around the PCs background as much as possible. My first session with players is generally all about background. I "interview" the characters, so to speak, asking about their families (who are they, what relationship did the characters have with them as they were growing up, what relationship do they have with them now, etc.), careers (were you trained or self-taught, does your work define you or is it simply a way to make ends meet, and so forth), and a dozen other factors (such as fears, hopes, aspirations, hobbies...). Basically, anything to flesh out the character and find out what sorts of things the players might like to go up against.
All this being said, I don't make the entire game seem like it was crafted just for that PC, because the element of surprise and unpredictability is just as important. However, if you have a Dwarven Ranger who hunts Drow and is afraid of heights, you probably don't want to have your campaign centered on fighting a harpy sorceress who lives atop the highest mountain crags...unless you're just evil.
So, I would say that the backgrounds of PCs are major influences on me as a DM. And I believe they should be. If you're running a pre-made adventure, for instance, you should try and tie something in the game to at least one of the PCs' backgrounds. It doesn't have to be blatantly obvious, of course, and you shouldn't play favourites, but the PCs should feel that the goal is important to them for more reasons than just gold and experience. Unless, of course, that ties in well with the character's personality.
I don't really like PCs with blank slates, because in my experience it never gets filled in. If a player doesn't come up with a background at all I still let them play, of course, but I tell them if they don't come up with some sort of background that they have amnesia...and I'm going to fill in their background for them throughout the game. I make no promises about what sort of person they'll turn out to be, either. So far, no one's been willing to take that chance with me...so they come up with some sort of background, even if it isn't very thorough.