Earlier quoted context omitted.
Email supports instant notifications, via your provider’s app or IMAP IDLE. I use Fastmail and get push notifications for email the instant it is received.
The point is if I turned on instant notifications I would go insane because they would never stop. I receive lots of emails and a lot of them are not very important or at least not urgent. While every IM I receive is something I at least want to glance at now.
Email notifications are flexible. I can choose to receive notifications only from contacts, search terms, or specific people.