Push Notifications - Help Me Understand Them

I understand how to set them up but I don’t quite uderstand how to use them.

For Example
I setup the notification to appear if a user visits a specic URL of the website.
User sees the Notification and presses the Subscribe Button

Brevo adds them to my contacts as an anonymous contact
As I don’t know anything about them, Name email address etc, I can’t email them promotions or updates so how do I use these push notifications to my advantage to try and convert them into customers?