The most common reason to put the username and password fields on different pages is to support both single-sign on (SSO) and traditional username/password login, but this design can be confusing for users. Separating these fields onto different pages may provide additional security benefits, such as making credential stuffing attacks more cumbersome and allowing conditional security checks. However, this approach may not be worth the added complexity unless it also supports SSO use cases. There are several options for handling both SSO and traditional login methods, including separating the fields into two distinct pages, using a single-page application lookup, or making the password field optional. Each of these approaches has its advantages and disadvantages, and there is no one-size-fits-all solution for designing the perfect login form.