c) Control Advantages
By continuously fixating on his hands (as well as peripherally on your environment), he may not risk tossing evidence or accessing weapons unnoticed.
PRO TIP: if you look away, even briefly, tossed evidence may be found later, but not in his legal possession; weapons can be injuriously discovered.
Trusting no one while looking for discordant body leaks may reveal his true intentions (study body language for pre-assault cues).
Adopting a position to his (strong) blind side makes it harder for him to assault you (a cover officer can approach on his opposite side) and will keep your gun-side away from him.
Using coordinated tandem arrest tactics is far safer than one-on-one situations as this minimizes risks of assault, hazards, and unexpected complications, etc.