[plt-scheme] class.ss: init vs init-field differences?

Hi. I'm studying the class.ss module to mzscheme. What exactly are the
differences between init and init-field? They seem to behave identically
in terms of object creation (with instantiate, etc). The only difference
I can see is that variables declared with init can't be used in
procedures (although why the heck would you want to initialize something
that can never be used??). Can anyone shed some light on the technical
and practical differences between the two?
thank you,
Tato N.