The most common reason to put username and password on two different pages is to support both:

1. single-sign on (SSO) (i.e. sign in with Google or a service like Okta)

2. username/password login

However, this login flow confuses people which is probably why you're reading this! Websites usually present a username and password field in the same view for us to log in. So you're not alone if you've ever wondered why the password field is missing or on another page.