Question

Why does an economist create a market demand curve?

Answer 1

The answer is to predict how people will change their buying habits when prices change.  The definition of a market demand schedule is, it is a table of the number of a good that all consumers in a market will buy  at a given price. So when there is a change in price, you will know what will be the demand for those products - you will know what their buying habits are if they buy even the price or vice versa.