The primary reason to do this is the same as why most logins will just say "incorrect username or password" rather than revealing the username is in fact, correct, and just the password is wrong. You don't want to reveal the identity or location of something that is hidden. If I have an unannounced project at github.com/ocdtrekkie/unannouncedproject, you might discover the name of my unannounced project by fishing for URLs that come up 403 instead of 404.
Confirming something exists narrows what someone looking to find out something secret has to look for. If you know what my username is, you only have to guess passwords for that username. If you know what my project name is, you only need to look for references elsewhere to that project name.
Just knowing the project exists could be telling, if your project was like... github.com/apple/nintendo-igameboyphone it would potentially be a really revealing thing if you hadn't announced it yet. Slack had a big fiasco when entering a fake @whatever.com address would let someone see whatever.com's teams prior to email verification. From just room names alone you could discern some potential acquisitions in negotiation, teams at Microsoft or Apple you might not have otherwise known existed, etc.
