[SamBam]

Tons of services send a verification link after registration, and when you click the link you are taken to a page that says "You're verified."

But in those cases there may be an automatic POST after you travel to the link, so it wouldn't be triggered by gmail looking up the url.

[crabmusket]

This may be for the purpose of ensuring the email address itself is deliverable. You don't want someone to sign up with random garbage, then try sending notifications, newsletters, etc. to it- I believe doing so can affect domain reputation.

For this use-case, it seems like even an automated link click would be a good signal of a deliverable email address.

[ziml77]

Not just deliverable, but also that it's correct. There's a lot of people who think that my {firstname}{lastname}@gmail.com email address is their own. If they try to register it somewhere, a verification email stops them from completing the registration.