Because... when worker put the paper on top of the food tray...
then he placed another food tray on top of that food tray... then
put paper on top of the food tray
and then he placed another food tray on top of that food tray...

Because the bottom of the food tray may not be clean. And I just hope
that I am not sharing with other people' germs.

Or do you guys have dishwasher to clean the food tray?
or are you guys using the dirty food trays over and over again... and that is why you guys put papers on the food trays...

You know that papers with advertisement on them and stuff.