I don't get it. On first boot my iCloud email is already setup, I add my other three email accounts (2 gmail, 1 self rolled). In the Mail compose window I have a dropdown that lets me choose any of the accounts to reply. I can also use Apple's new hide my email option.
Okay so you can proxy via Apple as well as choose from Alice@example.com, Bob@example.net, or Charles@example.org. What if you want to be Joseph@example.net today? In Thunderbird, you click on the From field and change the name, et voila. From the comment above, I presume that on macOS email, you will need to dig into settings.
My use for this is to give everyone a unique address (bitcointalk.org@lucb1e.com is one that was leaked to spammers already anyway). I block senders that send spam. No need to ever wait for spam filtering or check any spam box, because I get spam on maybe three addresses per year and those are easily dealt with
(Of course, your email server will need to allow using arbitrary sending addresses.)
Thunderbird lets you send mail from an account that doesn't explicitly exist. On your Mac Mail app, you need to sign in to three accounts to get three items in the dropdown (Even if you don't have to do it on every machine, you had to do it at one time). Thunderbird lets you make up the From: address on the fly.
They mean an arbitrary "from" email address, useful when you own the domain attached to your address. You can do that in Mail.app, but you need to configure it in advance, whereas Thunderbird lets you type any address directly.
I want this feature too because I use custom addresses, such as booking@mydomain.fr for hotel reservations, and replying with contact@mydomain.fr could be confusing to the receiver.
