I think when people talk about the importance of anonymity in communication, they aren't talking about the people talking to each other not knowing who the person they are talking to is. What they are talking about is the ability to talk to other people without a third-party (namely the government) knowing who you are talking to.

Pre-internet, this was accomplished in many ways. Secret meeting groups, anonymous newsletters, etc. With modern communication and data collection, it is much easier than ever before to know who is talking to who. If a government always knows who is talking to who, it becomes much easier to suppress detractors.