Just to chime in here by adding another use case.
Some websites allow you to enter multiple email addresses (I'm beginning to lose track of how many I have...!), and this is a convenience factor that helps to prevent accidentally creating multiple accounts where you need only one. Sites like PayPal allow you to do this, and in fact you can log in using any one of your email addresses, obviously the password applies to the entire account regardless. What is filled in automatically is often driven by a web site where you are making a payment, and the email address used can relate to what your email address is being used for (i.e. personal business vs. private vs. employer related expenses, etc.).
The password manager in both Google Chrome and Safari actually already handles this seamlessly and provides a popup to decide which username to use (and which password, if you have several username/password combos, which is a slightly different situation to having a single password but is supported as a side-effect).
Indeed, I don't use that feature for PayPal or other websites because it can be pretty confusing (and it doesn't seem to be mandatory), but it's worth noting that some sites do things that way for whatever reason.