The liberty we know as Freedom of Speech, when written into law, only says that your government cannot retaliate against your for speaking your mind. But your friends and family can, and would. When people have something important to say that can’t be said, privacy and anonymity are the liberties that make it possible to say it anyway.
There are many things you can’t say.
Even if you know them to be true, or at least genuinely believe them to be true, you still can’t say them out loud. You know you’d be in a mountain of aggression for saying them. The backlash from your peers would not be socially worth it.
What these things are varies from person to person. Somebody may question scientific consensus on a current topic. Others may have opinions on hotbed political issues. Others still may explore the spiritual (or lack thereof) in ways that would not be approved of among friends and family.
Whatever these things are, we run into the inevitable distinction between “narrow” and “rich” Freedom of Speech.
But before we go into this, let’s first establish that there’s no such thing as “limited” Freedom of Speech. There is never a “Freedom of Speech, except for”. If you don’t have Freedom of Speech for the most reviled subjects, you don’t have it at all. This is really a black and white issue: Freedom of Speech was specifically constructed to allow the most unforgivable to be spoken; to allow the utterances people feared most.
(Generally speaking, people fear neither the agreeable nor the harmless. There is no need for a constitutional protection for statements that people agree with, or that people find eccentric and harmless. Nobody needed police escort for claiming that kittens are cute, or that the moon is made from Emmenthaler