When one goes to alexa.amazon.com, one is presented with the ability to sign in as follows:
Email (phone for mobile accounts):
What is the difference between using a phone number to create an Alexa account and logging into iy vs. using an email id to create an account and then logging onto that email id on the Alexa app on a mobile phone?
So per an Amazon support chat, the Alexa and Echo/Echo Dot features available on an account where you login by phone number are the same as that available on an account where you login by email id. It's just a matter of personal preference.