There are some, many even, instances where the kind of fraudulent modifications you might want to do could not realistically be done at the time the page is saved, but could be done at a later date.
this tool does not guarantee that you've not modified the export before you stamp it. The main protection is the timestamp and the fact that the hash is calculated by the extension itself. This proof just guarantee that this particular file existed at this date. I personnally believe that even if it's not a siver bullet, the certain date is the main protection it provides. If you want to make some fraud with a bitcoin timestamp, you need a proper timing and preparation. In conclusion it just makes things harder
Being able to prove that a certain webpage exists locally at a given date is rather useless. The only utility there is if the page contains confidential information and you want to prove that you had that information at that time, but you could do that just as easily without saving a whole web page in the process.
Yeah, ironically the whole point of SingleFile for me is that I can locally edit webpages to strip out ads and other crap so I can send them to friends or family who might not have ad blockers.
So for that, it's useful.