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 nostr:npub1sg6plzptd64u62a878hep2kev88swjh3tw00gjsfl8f237lmu63q0uf63m has removed his as that account is verified by other means and doesn’t really tie to a commercial entity.