In an interview with the cofounder of the encrypted email service ProtonMail:
…because what is email fundamentally? Email, I would argue is actually the most meaningful passport of the 21st century. In the 20th century, your identity was tied with your passport. That was who you were. That was how you verified to the world who you were. That was how you accessed all the services that the world could offer you. Today, everything’s moving online, and every online service that you sign up for is actually linked to your email. It’s your online passport, and the idea of using your corporate email account doesn’t actually work because that would be like say, your employer is your identity. It’s not true. So that’s why for that reason, you know, that’s not really an option for most people… so Gmail isn’t just, let’s say controlling your data. They’re also controlling your identity in some ways. And you don’t realize that when you set up for it. But that’s what is happening. And that’s why we feel it’s very important to have an independent identity.
This is true even if you don’t actually use email to communicate much. All your friends might be on Facebook or Instagram or Twitter or Reddit or even Facebook Messenger or iMessage or Slack – they are all tied to email-based identities.
PS: in countries like India, that passport is mostly your mobile number.