To be able to drag a node onto a cable, (lets say an Image Type cable) and have the node auto-connect between the two nodes the cable is connected to if type matches.
For example if we had a
fetch image connected to a
render image window, dragging and dropping a
blur image node onto the cable between the two would insert the
blur image node between the two, and auto-connect the input and output correctly.
This is just a simple time saving feature.
Alternatively: it could also work if the cable was selected, that simply double clicking the node in the library would auto-insert the node between the two.