For myself, I think they tell you about the quality of an individual vote and the momentum behind a candidate.
There's a thing about followership which I learnt from a Ted talk. The most important moment in forming any group isn't the appearance of a leader. It's the adoption of the leader by the first follower. The first follower gives people permission to sign up. Once folks see this happening, they grow in numbers fast.
When Warren was doing those little meetings away from television, she was finding her first followers. It worked. Now she is reaping the benefits and gaining momentum.
That is what I think Warren's crowds mean. This isn't a money and advertising-manufactured tide. It's a sign of genuine support. The polls cannot spot this phenomenon, except in arrears.
I agree that she will have difficulty selling her plan to people with good private insurance. And to black voters who at the moment are supporting Biden.
But I think her taxing the superwealthy /anti-Wall Street message will resonate with blue collar workers, who are Trump's base.