Comment Re:Finally. (Score 5, Informative) 52
The reasons are even in the summary. If the other party has your phone number, they can more easily screw up and text you something sensitive rather than using Signal.
Consider that you are someone interesting enough to have a really skilled adversary with some resources. You're a journalist some powerful person or company doesn't like.
If someone compromises the person on the other end of your chats, they now have your phone number. They can now use your phone number for all sorts of phishing attacks, and have a much wider set of paths to attack *your* device. If they just have your Signal username, it's much less useful to them.
It's also often wanted for a much less technical sort of privacy. Women in particular give out fake numbers all the time, and many prefer to share Instagram and the like instead of a phone number. You can just block someone on Instagram, if they have your phone number, that at least feels much more permanent.