The callback allows setting back the original values before adding a new configuration for the video output (either window provider functions or output and video callbacks). There are some duplications between variable set by the callback to reset back to the old values and potential variable set by the new callback that would have overriden those, but it allows having a centralized callback instead of ensuring every configuration has been removed from everywhere and maintain this kind of list at every setter location.
This effectively allows setting vout, dec-dev and OpenGL implementations from the libvlc configuration.
VideoLAN code repository instance