Software diagram helps seeing the whole

Software engineers who write code without diagramming is like the Blind men and an elephant.

With the idea of "the whole is more than the sum of its parts", the whole can be pictured by the diagram, whilst the parts are the code you'd normally work on a daily basis.

Having different views of the architecture, such as a more physical view, or more logical view, helps you move around seeing the whole from the different angle. Movement increases accuracy of perception.