Look at this:
What do you see, a delicious ice cream with yummy syrup, isn’t it?
What if I told you, that the ice cream is nothing but a smashed potato and that syrup is a motor oil.
Yes!
During photo shoot for ice cream ads, their actual product i.e. ice cream can’t stand in front of heavy lights and those syrups are not actually photogenic.
If you try to shoot original ice cream in studios, you might end up getting a milkshake looking something in the bowl.
To present the products appealing, they have to use various non edible items which are photogenic and look way better than the actual food. For food items, they use props. For garnishing effect, motor oil, glue, cotton balls and even a shoe polish are used.
So basically, potatoes and motor oil are making your mouth watery.