Because at product launch time, 

the code delivering the new functionality is already in production. 

Months prior to the launch, Development has been deploying code into production, 

invisible to the customer, but enabling the feature to be run and tested by internal staff. 

At the culminating moment when the feature goes live, 

no new code is pushed into production. 

Instead, we merely change a feature toggle or configuration setting. 

The new feature is slowly made visible to small segments of customers,

automatically rolled back if something goes wrong.

(In The Phoenix Project, about DevOps)