[racket] prop:dict question

I was looking at trying to implement a structure that represents a
dictionary, but some of the elements of the interface don't make too much
sense to me yet: there are elements corresponding to ref/set as I expected,
but what I didn't expect were iteration as well..  Since there's a separate
protocol for sequences in the form of prop:sequence, the prop:dict elements
corresponding to count, iterate-first, ... seem superfluous.  Why does
prop:dict include these methods?

Thanks!
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