One I can think of (and I have to say it's probably a limited case, so perhaps add-on would be best) is if you're making something available for download and want to ensure certain members obtained the document.
Or if you're selling a style or something, you can just have XF manage the view/download rather than creating your own script to do it. It's a special case though so I'm not sure about having it default would make sense, but if it can with no real impact on resources it could enhance it just a bit.
What if your rules are in an attachment an you make it clear that all must grab the rules document or something else along those lines. Things like that are very obscure ways to see who actually is interested in keeping the community nicey nice and to see who is there to pillage and leave.