[racket] What are "impersonators" used for

Dear list members,

In the Racket reference under "reflection and security" they give the
functionality of "impersonators".  As far as I can see there is no mention
in the Guide.  I can see what they do, but I'm unclear what they are used
for.  Could someone give me a relatively simple example of where and what
they would be used for.

Many thanks,
Harry Spier
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