Because once you are up to speed the business and devs work more in concert, which lets things flow fast, especially if changes are small and many, rather than fewer and larger.
As devs grow secure in the infrastructure (”it kinda just works” from their end) as well as the ability to roll back within seconds, deploying things to production becomes no big deal.
The ”separation of concerns” and containerization enable some of this, but only part is tech — I find a great deal is rooted in the culture and people of the same mindset working together.
