It allows you to save a Google Slides presentation as a working html file using the browser's "complete webpage save" feature (Ctrl/Cmd + S).
The speaker notes window also works offline.
I made this, because if you try to save a google slide in presentation mode without this, the presentation will be broken.
Note that links in the form of
docs.google.com/presentation/d/*/present*, given that Google Slides opens them in a popup and Greasemonkey doesn't run in popups, you would have to copy the link and open it in a normal tab.
It also works for published slides in the form