The Southern economy depended greatly on cotton.
The South had an agricultural-based economy, whereas the North had a manufacturing and industry-based economy.
Farms and plantations took up much of the land in southern United States, and people made money there by growing crops and food. The major crop was cotton. It had many uses and was very prized.
People wanted a lot of cotton, so they grew more in their fields. They used enslaved people to pick cotton, so ultimately, the southern economy also depended on slavery. The basic idea as to why cotton was important is that many people liked it and it was a booster to the economy.