You’re on the right track, the immediate value of the email addresses is near nil. Instead consider these email addresses are highly self-targeted due to the nature of the sign up. That can then be sold for direct use, for resale, or most likely for inclusion in larger data sets that aggregate tracking information about individuals for ad targeting and browsing/results personalization. For example, consider the value to Amazon to be able to match this expressed interest with the email address of my Prime account; they could then push SQL-related content on my Amazon front-page with justifiable confidence I’d buy something vs the default of showing me canned kidney beans. Of course, that they show me canned kidney beans _because_ I just bought an SSD does show how this is mostly pie in the sky/skynet nonsense.