For the pouch of the thief to take effect your hero has to accompany the troops on the raid.
As you know the cranny will protect a certain amount of resources of the total villages storage.
E.G (using one resources only for simplicity's sake) if cranny is 2,000 of resources and the village holds 3,000 wood a raiding force would only be able to take 1,000 wood.
With the pouch of the thief equipped the hero would be able to dip into an extra 10% of that so they would be able to take 1,200 wood instead.
I hope this makes sense.
To fully utilise it look for local accounts with high cranny levels (this can be obtained by scouting). If you find one with 10k+ there is 4,000 resources only accessible by the pouch which you can have without other raiders touching.
It is very clear now, thank you very much!