To make a long story short, this feature lets you HIDE your email address when signing up for sites/services you REALLY don’t want to have your email address.
To give you a scenario: Say you see something on Social Media you MAY want buy, but you’re not sure. You KNOW as soon as you visit the site, that want your email address.
Instead of giving them your REAL address before you can even add the product to your cart, you can use the Hide My Email feature that comes with an iCloud+ subscription to create a email addy that forwards to your real email.
So AFTER you buy the product (that was just aiight), and IMMEDIATELY get hounded with emails on a daily basis about other products you don’t care about, you can deactivate the fake email address which will immediately cease all communications…because the email doesn’t exist anymore!
…which is SO MUCH MORE SATISFYING than finding the tiny ”unsubscribe” button, that you’re not even sure works.
So go ahead and use that feature more so Apple doesn’t kill it.