The territory serum corresponds to the "9x6x9" that most bees have - essentially, it defines the area which the bees use to find flowers and "defend".
This becomes important when you start finding bees with nontrivial effects - you might want to enlarge the territory of a bee which gives off an aura of regeneration, for instance, while you might want to restrict the territory of a bee which stings.
That said, there really aren't very many bees with a nondefault territory... I think the explorer branch might have a larger one. And anyway, an effect nullification serum is usually better for negative effects.
