Many important services in Google, e.g., Search, Ads, and Gmail, have dedicated teams of SREs responsible for the performance and reliability of these services. Thus, SREs are on-call for the services they support. The SRE teams are quite different from purely operational teams in that they place heavy emphasis on the use of engineering to approach problems. These problems, which typically fall in the operational domain, exist at a scale that would be intractable without software engineering solutions.
—— Google SRE Book, Chap 11.