Its a confirmation that the npub has a connection to a secondary website. It can be used as a call out, for instance i am wedgesocial@nostrplebs.com
The idea behind it is that if it said beth@ftx for the verification you knew they were affiliated with that company. You will notice jack (npub1sg6…f63m) has removed his as that account is verified by other means and doesn’t really tie to a commercial entity.