Blanket service levels are popular in some IT services departments
Because they are easy to understand from the points of view of both the provider and the to customer. They have all the benefits of mass production. They are relatively easy to produce, easy to train the service staff how to produce them, cheaper to to produce than exception-based services. They simply both service administration and the service level agreement itself. But they rarely reflect the reality of the real customer environment, which is why we tend to diverge from them at point of delivery