It used to take up to about 9 months to deploy or longer. With months’ worth of lead time in the run-up.
It’s marginally quicker to walk from Istanbul to Edinburgh than it was to make any significant updates to your application.
So some smart people sat down to figure out how to reduce deployment time. Reducing deployment time is incredibly powerful because it shortens the feedback loop, which means that the features that customers want (or don’t want) are built (or removed) as quickly as possible.