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Pirating can be established only with field fighters. When the battle takes place your field fighters attack the opponent field fighters, and if you win you get 1/4 from each resource if you have enough ships.

You can get maximum 1/4 from each resource. 1/2 is for normal attacks.

I believe the other advantage is that pirating does not send an SMS warning, even if your spy level is lower. The disadvantage, of course, is that you need a lot of field fighters to carry a worthwhile amount.