That’s for continuity
I always thought it was cause actors are on strict diets and are doing multiple takes
That's a factor (not that David Fincher cares, Max Minghella threw up because he had to eat a burger for every take of a scene in The Social Network), but it's mainly so that a character in a conversation doesn't go from talking to already having a fork in their mouth when it cuts to the over-the-shoulder other side of the conversation
It also means that they don't have to constantly fill the plates back up with food for more takes, or that the amount of food jumps around
That's a factor (not that David Fincher cares, Max Minghella threw up because he had to eat a burger for every take of a scene in The Social Network), but it's mainly so that a character in a conversation doesn't go from talking to already having a fork in their mouth when it cuts to the over-the-shoulder other side of the conversation
It also means that they don't have to constantly fill the plates back up with food for more takes, or that the amount of food jumps around
That makes sense
It always throws me off when the amount of beverage in a cup keeps changing