still confused about "order-only" prerequisites

[Robert P. J. Day]

  can someone give a practical example of "order-only" prerequisites?
i recall from once upon a time i asked about this sort of indirectly,
and the impression i got was that a lot of people really weren't quite
sure how these things worked.  and the online manual, while it gives
an overall, general explanation, doesn't supply an actual example to
clarify things.

  so ... any takers?  does anyone actually use these things in a
meaningful way?

rday