Thieving is used to steal various resources from chests and some NPCs.
Thieving will enable a bounty on you.

(Quantities below might not be 100 % accurate, as I've gathered the info myself with relatively small sample sizes.)

Level Type Possible Loot (qty) Exp. Chest
1 Chest Coins (11-80), Air Essence (6-11), Water Essence (4-10),
Earth Essence (3-6), Fire Essence (2-4), Void Essence (1)
10 Man
15 Man
20 Chest
30 Man
30 Guard