In the "Welcome Survey" epic, we added a survey to the account creation process so that we could learn a little about what each user is trying to accomplish by creating their account.
"Part 2" of that work: After the login is created, we can direct the new editors to help content that is relevant for their goals, or to WikiProjects that match their interests, or potentially match them with a mentor who shares their interests.
The engagement emails epic is one way the Growth team is addressing Part 2, in which we will be sending emails to newcomers to engage newcomers off-wiki. However, we can only email newcomers who have an email address, have verified it, and have opted-in to receiving this contact. We know from T204785 that only a minority of users in Korean and Czech Wikipedias have verified email addresses. Therefore, in order to increase our chances of making an impact with engagement emails, we want to do some work to increase the number of newcomers who we can send emails to.
Here is a Nielsen Norman post about best practices for asking users to supply information or give permission: https://www.nngroup.com/articles/permission-requests/
It has good recommendations for how to explain why having an email is valuable.