Raiding has to come with a sufficient reason for those raided to believe the raiding party already has at least a portion of the material, either obtained through interception or inside sources.

The idea is that the people being raided will hand over as much as possible because it’s in their own best interest - knowing that anything they don’t produce may be in the material already obtained, making them liable for destroying it.

A late and preannounced raid without any material obtained beforehand is useless.

(It should be noted that there is no need to actually obtain any material if you can make the people raided believe you have some of the material)