Difference between use-syntax and use-modules?

[Neil Jerram]

What is the difference between

(use-syntax (ice-9 syncase))

and

(use-modules (ice-9 syncase))

As far as the code is concerned, I know that use-syntax sets the
module-transformer of the current module, whereas use-modules doesn't,
and that the module-transformer is used in primitive-eval.

What I don't understand is what difference this makes.  In particular,
it seems that typing `(use-modules (ice-9 syncase))' (_not_
use-syntax) in the REPL is sufficient to permit defining and using
macros with define-syntax and syntax-rules.  So why would I ever need
use-syntax?

Thanks,
        Neil