The relevance of the letters is that they represent either an unknown quantity, or that they can stand for ANY quantity.
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the reason you use letters in algebraic expressions is because you don't know the number so a letter replaces it.
logic
There is no good reason: it is simply a convention.
No particular reason. It is simply a convention with no rationale.
It's the same reason that no letters in the English alphabet have a final form: it's just how the writing system evolved.