These are burner addresses, the vast majority of which I don't care about. If I ever wanted to move away, iCloud conveniently can show me each address, when I created it, and for what site. I could then change my address on the few sites I wanted to retain the account on.

I actually have a domain I setup with Fastmail just for burner addresses, but Apple offers enough additional functionality (easier to use, tracks the site I created it for and when) that I keep using Apple's offering.