These are a couple of reasons why a file card must be used to de-clog a file:
After constant use, the teeth of a file will become clogged with small bits of metal, particularly when working with soft metals.
Some examples of soft metals are aluminium, lead, copper, silver and gold.
If you do not de-clog the file, it will scratch the metal surface you are working on.
Another problem is, if your file is clogged then it does not work effectively.
When the teeth of the file become clogged, this levels the surface of the file and reduces its cutting ability. The file then slides over the workpiece, hardly removing any material.
Wood files can also become clogged with sawdust when filing wooden workpieces.
The sawdust is soft and therefore is easily removed when the file card brush is applied to the teeth of the file.
How to prevent a file becoming clogged
When filing soft metals, to prevent a file from clogging up, rub the file with a piece of chalk to fill the teeth.
This reduces the clogging of filings and other materials, making stubborn bits easier to remove. The chalk does not prevent the file from doing its job as the chalk is soft and acts like a lubricant.