Not to get metaphysical here, but erasing is more than just a simple action to an artist. This may sound like bull, but especially to a budding artist, there is a lot of implication to the simple action of erasing a mark from your paper.
Why? Erasing has a lot of connotations attached to it. What we've been taught since we were little is that erasing is the sign of a mistake. We erase due to an error in our judgement or skill. For an artist this is one of the most difficult things to overcome, and make no mistake, it must be overcome to be a good artist . Erasing is as much a part of the drawing process as mark making, and should be incorporated into the process as such.
One of the most difficult parts of erasing that I've always struggle with is the idea that I'm losing ground. When I look at an image I'm creating I worry that I won't be able to reproduce a section that needs erasing. Sure, that section is not what the image needs and should be removed, but what if what I replace it with is worse? What if what I erase was the best I could make in that particular part of the image? What if I ruin it?
Eventually I erase, and few times, if any, is the result of my erasing worse that the original. In the off chance that it is, I erase that and draw again. If you're every going to progress in any art form, or life for that matter, you have to be willing to remove what you've done permanently and rely on your instincts and skill to replace it with something you know will be better.