I do have the G752VY and I put the rubber plugs back in the box and left them with everything else like the original drives etc.
They are not essential. The intent is if you are not using the machine for an extended period you are supposed to put the plugs back in where you took them out. They seal up the dust port on the fans and are intended to keep dust out if the machine is sitting up for a long time.
Dont worry, you are not the first. Most question WTF they are for and either chunk them right out of the box or lose them shortly anyhow.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein