What is twitter:domain good for?

Question

I'm currently trying the Twitter Card Validator for my website.

I did not set twitter:domain, because the help text shows

RESTRICTED - twitter:domain will be automatically generated from your card's URL.

Not required

But I've just wondered if I should set it anyway.

What would be the effect when I set twitter:domain? To which value should I set it? Should I set it at all?

(I guess martin-thoma.com, but it might also be http://martin-thoma.com or martin-thoma.com/.)

Answers that don't cite a reliable source are note acceptable!

Answer 1

According to this, twitter:domain is used to display the text on the button "View on ...". The validator seems to agree. Once you enter it and update the card, it changes from "View on web" to "View on (value of twitter:domain)".