Benefits of getting user registrations?

what r the benefits of having users register on your black hat websites? I could enhance their user experience and I guess use email marketing. What else could I do profit wise? Is it practically useless to enforce users to sign up?

More the users, more will be the interaction on your website. It is a positive sign and means that more advertisers will be interested on advertising on your website.

Why they're registering on your website?

If it's a free trial, you can follow-up with them to purchase.

If it's to access the access the content, you can upsell them to by a high quality content.

The ideas are endless, it depends on why they're registering on your website.

Give more details to get accurate answers.