What’s the point besides a curious hack?

Does there need to be one? It raises awareness of the headers used, even if they’re used weirdly here.

It does not really work on all browsers, but I can imagine scenarios where in some network middleware you have some dynamic control over headers but don't have control over the content, so that might be once place something like this could be useful.

One could leverage this as a clever way to update something on a page without redownloading it. Or from a proxy, without having to decompress & process body. Unfortunately support is limited to Firefox, so it's useless.