Betting on that there are more than one of those people who don't speak up out there, I wanted to tell those people who are such great listeners that if you could even restate in your own words what you understand that someone else is saying, that will let them know what it was
they said that reached you. Often when we talkers/writers write something or talk, we don't really know the effect on others - or what exactly brought about that effect. We're just communicating and hoping it reaches someone somehow.
I see agreement as a a signal of resonance between people - that someone is following trains of thoughts and the associative jumping around that people can do. Lots of times in a dialogue group, a subject is being talked about and you'll want to say something, but if you hang back you'll find someone else bringing up the same point.
Another reason to speak up is that in some situations, people will come to the same conclusions from completely different circumstances. It's always fascinating to know what those specific circumstances were, for me at least.