Because of the ongoing fucktastrophe, the cries of "Use SIGNAL!" are constant and unavoidable. And I get it, it may be the least-bad option in a sea of terrible options. If, that is, you choose to ignore the advice of "don't use your phone for that shit" (the Stringer Bell Rule). But out of curiosit...
The big upside of signal is that it is better then SMS, and has more adoption then any of the other reasonable options. Adoption is still not enough to make it that useful when compared with Messenger and SMS and even with this addressbook thing your complaining about trying to drive it.
Big downsides abound too including needing a phone number, and being tied to a phone.