An analogy is writing everything on one piece of paper with a pencil. When you need to change or remove something, you cross it out, instead of erasing, and write the new data to a clean part of the paper. When there are no more clean areas, you use the eraser to erase a crossed off section.
The larger the paper, the less frequent you come back to the same area again with the eraser.
Using an eraser on paper slowly degrades the paper until that section tears and never gets used again.