[racket] init and init-field

Dear list members,

The examples in the Racket Guide on Classes and Objects use something like:
(class object% (init x)
  (define current-x x)
........
.........
The examples I've seen so far don't use "init-field" which would
eliminate the need for the internal define .

Can someone give me an example where its necessary (or even better) to
use "init" instead of "init-field"

Thanks,
Harry Spier