In Scheme, what's the point of "set!"?

What's the point of using the set! assignment operator in scheme? Why not just rebind a variable to a new value using define?

> (define x 100)
> (define (value-of-x) x) ;; value-of-x closes over "x"
> x
100
> (value-of-x)
100
> (set! x (+ x 1))
> x
101
> (value-of-x)
101
> (define x (+ x 1))
> x
102
> (value-of-x)
102
>