There are a few reasons. Probably the biggest one is that it can make it easier to determine how your login should be authenticated.
For example, when you login to a Microsoft account, it first asks for your username. Once you enter that, Microsoft determines if it’s a personal or business account to redirect you to the appropriate login method.
It can also be used to first determine whether you have multi-factor authentication enabled or if they may need to present another method of validation (i.e. CAPTCHA).
Latest Answers