Hey there, dubmann2,
Thanks for asking about the iCloud Mail option in System Preferences here in the Apple Support Communities. We're happy to provide you with some information!
The iCloud Mail option is not a backup for your email, but rather an opportunity to use iCloud as your email service. If you enable it in System Preferences, you will be able to select an @icloud.com email address if you do not have one, and will add it to the Mail app for your use. If your Apple ID is already an email address that ends in @icloud.com, @me.com, or @mac.com, it will add it to the Mail app if it isn't already there. You can also access your iCloud Mail at iCloud.com.
You can get more information on iCloud Mail in the following Apple Support articles:
What is iCloud Mail? - Apple Support
Using your @icloud.com email address - Apple Support
If you would like to back up any information on your Mac, including email, you can use Time Machine: Back up your Mac with Time Machine - Apple Support.
Have a great weekend!