<< Why is it important to have an apple id password, and a Mac login since they seem interchangeable? >>
[in my opinion] they are NOT interchangeable.
Use of your Apple-ID requires an internet connection for validation. Its password is stored in your Apple Account on the cloud, ONLY.
On a device that is inherently connected, such as an iPhone, using Apple-ID is not a conflict, and you get access to the device itself with your locally-validated passcode, Touch-ID , or Face-ID.
On a stand-alone computing device like a MacBook Pro, such a device can be used extensively without a daily internet connection, because it has a locally-validated encrypted login credential. Requiring an internet connection to do ANYTHING on a Mac is not appropriate.