What’s an acetate used for in regards to comics?

I was just wondering what an acetate is in reguards to comics. On ebay I saw an acetate which was a transparent film thing with the cover of a comic printed on it in black, No color or anything. Were these used in the creation of the comic? Are they valuable? Is it something I should look for when collecting original comic art? I really have no clue what these are used for..