I’ve updated the link, the slides are available here:
https://www.slicer.org/w/images/7/79/SlicerModulesProgrammingBeyondBasics.pdf
Parameter node stores your node and parameter selection so that if you save the scene and reload it you can just click on Apply to run the processing again.
Parameter nodes allow you to set up complete processing pipelines using CLI modules (select the output of a CLI as in input of another CLI). You just load the scene, select data set as input, and the pipeline is re-executed automatically and you see the output as soon as the processing is completed. It only works for CLI modules though (and for loadable and scripted modules that has auto-run/auto-update feature implemented).