Privacy

How so?

Signal also uses our metadata encryption technology to protect intimate information about who is communicating with whom—we don’t know who is sending you messages, and we don’t have access to your address book or profile information. We believe that the inability to monetize encrypted data is one of the reasons that strong end-to-end encryption technology has not been widely deployed across the commercial tech industry.

Source: https://signal.org/blog/signal-is-expensive/

I haven't verified that claim investigating the source code, but I'm positive others have.