iMessage with emails and can't use icloud emailbox

Can someone give me more detail or reason why adding an email let's Mac users use iMessage. The reason why because my phone is damaged. My Sim card is not in it anymore so I was unable to send messages on iMessages, but I had a friend tell me will just add an email. I thought my iCloud that is associated with the iMessages was the email she was talking about and you know it didn't let me use it. I couldn't sign in because when you go to settings and you go to account you can enable the iMessages to your phone number or email. Yesterday my husband told me to add my Gmail account in the settings and in the sign-in security, which I did and that enabled me to now messes people on iMessage on my Mac. I would just like to know the reason why the enable that and how come it did it?

Since I don't have a phone currently and my Sims card is not registration anymore cause my phone is damaged. How do I get my iCloud mail back? The reason I ask is that I can't receive any new emails through my iCloud. I noticed I can send them, but they can't be backed up into my iCloud because I have to get more storage, but the funny thing is that I enable my email my phone number to my iMessages and that used a lot of my data I assume not too sure so I was unable to send and receive emails so I started to delete things and try to takeoff my number, but I couldn't. I was just like help on getting my email back on.... trying to put it back together. It's been a rough month.