[zero_by_divide]

The only justification I've ever seen was places gearing up for multi-factor authentication. The front login page, asking for your account, tells the backend how you're configured to login (token, password, biometric, whatever). Then the second page is variable depending on your authentication method.

[BrandonSmith]

Some authenticate (Google comes to mind) allows for delegation to third party scenarios. So, similarly, knowing the account is necessary to determine the next step.

[ktta]

Is that the only real reason? blockchain.info immediately pops up a 2FA input bar depending on the previous inputs, not a second page.