There are some online services that use email login. This means that instead of a combination of user name and password, only the email address is entered and a login link is sent to it. Basically, this is a good option to increase security a bit. The service only needs to store a list of email addresses instead of the corresponding password (hopefully encrypted and hashed) for each user.
But somehow this is also quite annoying sometimes. Instead of simply having my password manager fill in the appropriate text fields in my browser, I have to retrieve my emails, possibly wait until the email is sent and received and then click on the link in the email. This considerably increases the effort for logging in and requires a few more clicks. And if I use for example my work computer, on which I have set up my password manager in the browser, but have to log in to the mail web application to retrieve my emails, then the whole process takes even longer.
And not to forget: The deletion of the login mails, so that the mailbox won’t get cluttered, adds to this.