Poisonous weapon state:
Requirements You are wielding a piercing or cutting weapon and you have a free hand.
You apply a poison to the required weapon. If your next attack with that weapon before the end of your next turn hits and causes damage, apply the effects of the poison, provided that the poison can be released by contact or injury. If you fail critically in the attack roll, the poison is wasted normally.
Special During your daily preparations, you can prepare a series of poisons of simple lesions equal to your rogue level. These poisons deal 1d4 damage per poison. Only you can apply these poisons correctly, and they expire the next time you prepare.
The first benefit is obvious, you get a free stock of poisons every day equal to your rogue level. That is some money saved there.
Now let's look at the rules for alchemical poisons, they say:
The application of alchemical poisons uses Interact actions. A poison usually requires a hand to pour into food or disperse in the air. Applying a poison to a weapon or other element requires two hands, with one hand holding the weapon or element. The use input for a poison indicates the number of hands needed for a typical application medium, but GM could determine that the use of poisons in other ways works differently.
If we look at Black Adder Venom, we see:
Use held in 2 hands; Bulk L
Adder venom is a simple but effective way to improve a weapon.
Salvador shot DC 18 Fortress; Duration 3 rounds; Stage 1 1d8 poison damage (1 round); Stage 2 1d10 damage per poison (1 round); Stage 3 2d6 poison damage (1 round)
Activate To interact
This means that 3 actions are taken to apply the black adder poison.
Instead, the poisonous weapon allows you to apply a poison in one action. This is a great improvement when using poison in combat, as it allows you to reapply poison efficiently while fighting.
Therefore, the benefits of the poisonous weapon (as far as I can tell) are:
- It provides you with a daily poison group for free injuries.
- It allows you to apply poisons as a single action.
On the subject of the questioned line.
If your next attack with that weapon before the end of your next turn hits and causes damage, apply the effects of the poison, provided that the poison can be released by contact or injury.
There is no difference in the results of an attack between the poison applied with the Venomous Weapon feat and the normally applied poison.