Kneaded Erasers are good tools to have but they lack the stickiness of Blu-Tack.Basically, if you form a point in both then touch the point of the kneaded eraser onto an area of graphite and lift it straight up, you will see very little difference. Repeat the exercise with Blu-Tack and you will see that a round area of graphite has been removed. Blu-Tack requires no mechanical action (such as rubbing) to work - a touch is enough - which is why it can fade an area while maintaining the detail.
I haven't used kneaded erasers (which I call Putty Rubbers) for many years but I still remember how they operated.
I should stress that although Blu-Tack is sticky it is not so sticky that it will leave remnants of itself on your drawing. This just doesn't happen. It's this stickiness that you turn to your advantage. If you want to progressively fade a small area, for example, you would dab or roll a piece of Blu-Tack over the area then repeat using the same face of the Blu-Tack. The face will become progressively dirtier as it lifts the graphite so the fading of the area of your drawing will become more and more subtle. Blu-tack really gives infinite control.
Don't regularly throw Blu-Tack away. Older Blu-Tack possesses different qualities from new. It will be years, rather than months, before you need to throw it out. To clean it simply stretch a ball into a long rope (or just pull pieces off) then squash it all back into a ball again.
Sorry, I've forgotten the question 
and I'm getting carried away.
Oh, perhaps I should tell you that I have vested interest in Blu-Tack because I sell it from my site - but none of the above has any bearing on this 
MIKE
www.SibleyFineArt.com
www.Starving-Artists.net
hehehe