There are many great arguments to protect truly private communications from a human rights perspective, and specifically through a Constitutional lens -- restoring the privacy of having a conversation in your living room and having your personal records stay personal are core first and fourth amendment rights which have suffered greatly in the digital age.
My work takes me around the world to support journalists, human rights activists, and a wide variety of amazing people working to improve the world. They are all facing incredible threats posed by powerful actors. These adversaries use malware, hacking, and all forms of digital attacks to compromise the networks of activists.
Open source, trusted, strong cryptographic tools -- and increasingly, trusted commercial systems such as Google's -- are their only available defense, in situations where failure can include targeted harassment, indefinite imprisonment, torture, and even death.
Encryption saves lives.