There's absolutely no benefit to it other than manipulating objects that might be around.
The first paragraph cuts to the heart of the genre relevant "swashbuckling" aspect of the martial form. However, this bit is abjectly wrong.
I really don't want to get into an SCA or Olympic/competitive fencing breakdown, but there is historical record of the "soft martial art" featuring an agile blade in one hand and an empty offhand from Saber, to Rapier, to Kopesh, to Short Sword, to Sword of Mars, to Scimitar. A great many armies from the Huns, to the Spartans, to the Romans, to the Americans/French/British infantry of the last few centuries (amongst others) employed agile sword with empty hand (without shield or main guache/parrying dagger) to great effect on the battlefield.
Two-Weapon and Sword and Shield are "hard martial arts" that map to the "hard" forms inherent to Karate, Taekwondo, Dirty Boxing. Two Weapon is something of a hybrid soft (Kung Fu) and hard (Karate) components contingent upon the studied weapons. However, only One handed and Empty hand is a fully "soft martial art", mapping to the defensive elements of Aikido and Kung Fu with the "lunging attack and escape" elements of Karate (see Leodo Machita) . There are distinct advantages to the style that Great Weapon, Two-Weapon, Sword and Shield do not offer:
- Considerably smaller target. While this form allows the user greater freedom of stance and corresponding smaller target area, the other forms are premised upon the user being squared up with the opponent in order to maximize the "hard" aspects of the forms (blocks and "hard parries".
- Greater strategic and tactical mobility. While the other user is burdened with weight and the requirements of balance of an off-hand implement and the more rigid requirements of "squared stances", this "soft martial art" is considerably more free in its mobility.
- Variance of postures and fluidity of stances provides angles of attack and escape that the other forms inhibit.
- The "off-hand" is very much more than useless. It is used to balance, feint, parry and grapple...and it is effective in doing so.
- Ease of use. It is the most intuitive as most people are not ambidextrous and readily grasp, and can therefore attain higher proficiency within, the concepts of "freedom of movement" married to "strong hand" techniques.