It's a high risk market. It's difficult to plan production. If you operate entirely on the cash market, you never know if you will make any money until your crop is produced. Even if you store your cotton for later sale, you are still speculating unless you have forward priced with cash contracts or hedged in the futures market. Cash contracts let you know in advance if you will profit or lose on your crop.
Exact price is known in advance.
No margin money is required. It'simpler than hedging in futures market. You have reduced flexibility. Price is usually below futures price. The futures market also lets you price your crop before delivery.