Why are unused variables bad?

Question

I would like to know why an unused variable is bad.

Is it because the compiler would create a bigger binary? If yes, is there a tool/script which can add an unused keyword or something like that?

Answer 1

The compiler gives you warnings to hint on things that could potentially be a problem or unintentional.

Unused variables will be optimized away most likely. But maybe you intended to do something with them – and in that case the compiler helpfully notes that you may have done something you didn't want.

What's the use in a variable you declare but neither read from nor write to?

Answer 2

Because...

Perfection is reached not when there’s nothing left to add, but when there’s nothing left to remove.