Results returned by check.torproject.org may be false:
Apart from the possibility of getting false results by accident the explanation above points out that a malicious network could try to deceive you by providing false results on purpose.
So why would you do a check with check.torproject.org? If you can’t rely on it what is really its purpose? How is it ever at all useful?
Earlier versions of Tor browser opened check.torproject.org on startup so there must be some kind of rationale behind it.