What exactly does "pirating" do?

[TeRMiNaTe]

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.

[TeRMiNaTe]

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

[Seyhall]

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.