With this command, you make preparations to hunt someone down and kill them. The main benefit of this is that if you bring them to the brink of death, you will have the options of looting them, and possibly granting them mercy. You should not use this command to protect allies or friends from being accidentally killed. For that reason, when you hunt someone, you have to log a reason. This does not mean you have to have a good reason for hunting someone, it is just there to remind people that the command is logged and they better not be using it to save friends or unintended victims from spells, magic items, or other attacks.
Be warned, sometimes you might not be the actual deliverer of the death strike, which means you won't get the option of granting mercy. There is no guarantee that just because you are hunting someone you will get the option to grant them mercy, loot them, etc. In other words, don't hunt someone unless you are prepared to deal with the possibility of slaying them.