An interesting side effect of prompting credit card sign-ups is that the visitor is engaged in getting use out of the service immediately. They're running again

An interesting side effect of prompting credit card sign-ups is that the visitor is engaged in getting use out of the service immediately. They're running against a ticking clock (14 or 30 days) to get whatever service running. Those who don't prompt for cards are under no immediate time pressure and therefore less likely to engage a month down the line when the specific problem that caused them to search for you in the first place is no longer as acute.