sometimes you need an unused variable. some uses in rust:

// destructuring
let (width, _height) = get_dimensions();

// trait implementations (i couldnt think of a better example for this)
impl Into for AlwaysZero {
    fn into(_value: Self) -> {
        return 0;
    }
}

// some types (eg. Result) must be 'used'
// assigned to a variable if we dont care about the return value
let _ = returns_result("foo");