The multi-part income statement still takes revenue and subtracts expense to determine net income, but it adds a step it first subtracts Cost of Goods Sold from revenue to calculate Gross Margin (also called Gross Profit). Why do we care about the Gross Margin? What does it mean? Why is it significant (or not)?

If Basketballs R Us buys basketballs for $6 and sells them for $10, how much does their net income increase if they sell just 1 additional basketball?

Explain (in three or four sentences) the meaning and significance of Gross Margin.