I think this is overly broad. If I have a chat webapp open, I would like a notification rather than having to manually poll it for new messages or missing messages.
A plugin works for notification.
It could be a useful feature, but marketing ruined it, just like everything else web related.
No it's useful, however it's has been incredibly abused by bad actors, marketing corps, spammers and co who sell their notification channels to third parties and is basically used as an AD pop up.
I use a web based phone services so I do like it in some contexts. Inbound calls ring up more visibly this way.