YouGov uses the shared data to generate insights for companies looking to understand consumer behavior. They combine your data with information from other users to create anonymous insights. According to YouGov, "The insights we get from this project are generated by combining your data with data we get from other members. These insights are completely anonymous, meaning that we will not share any identifiable data with our clients."
It’s entirely up to you to decide what data you’re comfortable sharing and whether the payments make it worthwhile.