As a result the payment in not only depends on our participation rate, but also depends on others. So there is a competition between users and every campaign will lower participants can be a better one.
Please let me know whether I am right or not?
Yes, you are right, if campaign is paying in stake, than less signature participants mean more tokens for you.
Another thing that I really can't understand is how will be get paid?
Most of these campaigns ask our ETH address. Why ETH address?
You will receive tokens into your eth address because they are smart contracts which are running on ethereum platform.