Question

If unary +/- operators are used to perform conversions as the Number() casting function, then why do we need unary operators? What’s the special need of these unary operators?

Answer 1

The Unary + operator converts its operand to Number type.
The Unary - operator converts its operand to Number type, and then negates it.
(per the ECMAScript spec)

In practice, Unary - is used for simply putting negative numbers in normal expressions, e.g.:

var x = y * -2.0;

That’s the unary minus operator at work. The Unary + is equivalent to the Number() constructor called as a function, as implied by the spec.

I can only speculate on the history, but the unary +/- operators behave similarly in many C-derived languages. I suspect the Number() behavior is the addition to the language here.