One of the things I have been observing as I learn with my kids is that the act of erasing plays a significant role in their learning process.
For example, learning to write with a pen, where every mistake seems irrevocable, is very different than learning to write with a pencil, where mistakes can always be erased, though perhaps not without some effort and frustration. It is a different thing again to learn writing on a chalkboard or on a whiteboard (as we do), where erasing is not only possible, not only effortless, but also a structural part of the medium, where it is within the normal function of the medium to become full and to be erased. It is a still different thing to learn writing on the computer (as we do also), where things cannot only be erased, but can be cut and pasted and copied and formatted and whatever else.
When something can be erased, it allows us to experiment, to try and fail, to account for the repetition and error that is a part of learning. Learning is process. It is not an attempt to create a product that is concrete and unalterable, but an attempt to create something that will enable us to go still further, to take a next step, to supersede what we have learned already by incorporating it into something new. This is why we should be less concerned with whether students can produce finished and polished products like essays and science projects, because these things are only valuable very provisionally, as trials and attempts and ventures and experiments that should merely mark a single point in a long trajectory of learning, a point that is not really more important than any other.
This is why it is important to allow learners to erase, to show them that erasure is not a sign of failure but of growth, or perhaps better, that erasure is a sign of failure being turned into growth. I want learners, my kids especially, to write as though they are scratching in the dirt with a stick, as though they are etching a wax tablet with a stylus, as though they are counting beads on an abacus, where their errors can immediately become the basis for the next step in their process of learning. I want then to be able to learn by erasing as much as by creating.