I've read The Hobbit by J.R.R.Tolkien in Spanish but trying to answer a question about a possible typo in another StackExchange forum, I've found this text in its English original preamble
Runes were old letters originally used for cutting or scratching on wood, stone, or metal, and so were thin and angular. At the time of this tale only the Dwarves made use of them, especially for private or secret records.
I thought that in "use for", the verb use means
to put something such as a tool, skill, or building to a particular purpose
So if we used a knife to cut the steak, that knife was used for cutting. But I don't get how a letter can be used for cutting or scratching. I know that Tolkien was pointing that runes were carved on those surfaces using tools, something like
Runes were old letters that originally used to be cutted or scratched on wood, stone, etc.
but I haven't found evidence of that equivalence in online dictionaries. Maybe you can offer me some links or extra information about this.