I guess many people hate them, but subscribe without thinking about it and don't know how to unsubscribe. For the websites I would expect them to be quite useful, because they will get a lot of extra traffic and returning customers. Even if people don't like the volume of notifications, they will still click on them if the headline sounds interesting enough.

Bonus answer: I think they are quite useful for web applications (mail, calendars, slack etc) and I usually prefer enabling notifications to installing a native client.