Consider a new website in development where it requires the users to register for a service.
Lots of sites require a confirm by email and by having the user enter captchas. What's the usage of both these devices?
If a site has a captcha verification, does it still need an email confirmation?
The captcha tells you that a real person registered, as opposed to a "bot".
The confirmation email tells you that the user entered a real, legit email address that they have control over.
Depending on your needs you may need one or both of these.