Provide a UI that anybody can use to paste in a URL to one of their posts and see how it will be rendered on my site if I reply to it
It answers the "how do I benefit from this work?" question about adding h-entry
Acts as another h-entry validator aside from running a parser manually or inspecting the JSON output of php-mf2
Could also act as the equivalent to the "Preview" UI in traditional blog-specific comments user interfaces which allow you to preview how your comment will look on the original post before you submit it, except in this case, it's before you send a webmention.
One could even imagine this as part of a cross-blog preview reply protocol, where a preview button on your own reply authoring UI would show you what it would look like both on your own site, and via an iframe, how your reply will be presented when syndicated as a comment on the destination (the thing your reply is in-reply-to). (Feel free to move this to a separate brainstorming page independent of p3k, just felt like it fit with this thread here for now. -t)