Why were food surpluses necessary for civilization to develop?

Answer 1

In order to answer your question, let me give a simple scenario:

You have a group consisting of 10 people. All they do every day is farm and farm and barely harvest enough food to survive. All they think about is their next meal, nothing more, nothing less. Each person makes enough food only for themselves. Now, let's say that one of people in the group learns how to grow more food to feed not only himself, but another person. He can feed two people now, so that second person no longer has to worry about where his next meal is coming from, he can focus on other things, such as making weapons and becoming a soldier, train to become a priest, or any other aspiration. Let's say he decides to become a soldier. Now this one person can defend his tribe against an attacking tribe. Now the people have a sense of security and in turn, they will start to grow more food. Since there is more food available, more *cough cough* babies will be born because they decide that "Hey, that lady/guy over there looks good". Eventually, you will get to the point that one person can grow enough food the feed 100 or more people. The other people that aren't farmers are something else, such as soldiers, priests, scribes, merchants, etc. The leadership positions (priests, scribes) organize the society and begin planning out the systems used for agriculture and raising an army. That is how civilizations are born.

Answer 2

It wasn't, the only reason why they had a surplus of food is because they didn't want to worry about food supply in the winter.