Help me understand common lisp macros

What can common lisp macros do that can't be done by a function that uses eval and gets quoted arguments?

I mean you can define

(define (myif pred ifstate elsestate) (eval (list 'if pred ifstate elsestate)))

and then call it like so:

(myif '(> x 1) '(+ x 3) '(- x 2))



so what else can lispmacros do?