Because it would save money, presumably. For example, in a 10,000 ton boat: "While the kite was in use, the ship saved an estimated 10-15% fuel, $1,000 to $1,50

Because it would save money, presumably. For example, in a 10,000 ton boat: "While the kite was in use, the ship saved an estimated 10-15% fuel, $1,000 to $1,500 per day." (From the linked article)