It's for contact discovery. It's actually pretty similar for email? If you enter an email address in your mail client and send an email to it, in most configu

It's for contact discovery. It's actually pretty similar for email? If you enter an email address in your mail client and send an email to it, in most configurations you'll get some kind of notification if the recipient doesn't exist. Email, of course, has an unlimited number of possible addresses. Phone numbers are a dense space with limited parameter length. So it is easier to enumerate all phone numbers.