A party with a thief has a trap-finder/disarmer, someone who can scout ahead silently, and someone with a chance of being able to use arcane and clerical scrolls when there isn't a magic-user or cleric around. But most importantly is the core Thief ability to Circle During combat: the thief takes advantage of distractions his allies provide in order to circle around behind a foe and attack. This isn't a backstab. It doesn't matter if the foe knows the Thief is Circling. The thief gets a +2 to-hit bonus from behind the foe, and upon a successful hit inflicts double damage. If for some reason the target is unwary or unsuspecting, the to-hit bonus rises to +4 and triple damage is inflicted.

When may the thief Circle? When an ally has dealt damage to the thief's target earlier in the round. Always double (Circle) or triple (Backstab) base die/dice damage first, before adding damage modifiers for Dex, item bonuses, etc.