Traversal in insertion order for Set.prototype.add/delete

[Qantas 94 Heavy]

Maybe my brain's just fried, but is there any reason for Set.prototype.add
and Set.prototype.delete enforcing iteration "by original insertion order"?
There aren't any side effects that I know of by iterating through the list
of elements in some other order, as all the elements of a set are meant to
be unique. Am I missing something?
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