When a shader node is used as part of a larger shader, it generates source code that performs a shader function call. The called shader will set its own global variables, but won’t set global variables of the caller. Instead, caller’s local variables can be explicitly bound to the global variables of the called shader using output parameters in the shader call.
In the VOP network, the shader node usually has only one output connector that represents the shader type. So, local variables bound to the globals don’t have corresponding output connectors on the shader node. This node allows wiring the shader node as an input and has global variables as outputs, making possible to access global set in the shader.