For example, we select 10 nodes in active material LightPollution and then also select the ScnMat as Emission, finally we click Add ActiveMat to ScnMat operator, the addon will copy the 10 nodes and their links automatically to Emission and put on the top of its upmost nodes.

Through MixShaders user can build a systematic directories for organizing procedural materials and the sub directories can be organized in a hierarchical way by end of use, similar functions, similar nodes etc. 

The 2nd use of MixShaders addon is to mix shaders automatically in the Shading workspace and the addon can automatically calculate the right position to append the material nodes accordingly.