Transactional email fails silently. Password resets stop arriving, receipts bounce, and the first signal is usually a support ticket from someone who could not sign in.
Delivery webhooks make that failure loud. A spike in bounces or a spam complaint is the sort of thing worth knowing on the day it starts, not at the end of the month when someone opens the analytics tab.
An address that will never work again. Worth catching before the retries pile up.
The event with the longest tail: complaints damage deliverability for every future send.
Usually the first visible symptom of a provider or reputation problem.
Each of these sends the events above. Setup is the same for all of them.