They don't sell advertising, they sell data. Primarily, leads for sales teams. If you want to start a new payment processing service, you subscribe to BuiltWith and they'll give you the contact info of the decision makers at the top 10,000 websites that use Stripe or whatever competitor you choose.

Got it, my follow-up question is, how is it acceptable for customers to have their data to be freely sold as "leads" to random 3rd party companies? Is it in the BuiltWith terms or something?

They're crawling and packaging already public information. If you don't want anyone to know you're the CEO of StartupCorp and your email is ceo@startupcorp.co, don't put that on your LinkedIn and the footer of your website. The business value is in the convenience of the packaging, not in revealing secrets or something.