The Thief job is one of numerous jobs in Final Fantasy III. When the player will get this job varies depending on the version: In the 2D versions, the Thief is unlocked alongside the rest of the Fire Crystal jobs. In the 3D versions, the job appears alongside the Wind Crystal jobs, making the job selection a nod to the original Final Fantasy.
As far as physical jobs go, Thief is high-speed, low attack. The primary stat of interest for Thieves is the agility stat: a stat which is naturally higher than their contemporaries and is further increased by knives: their primary weapon type. This agility means that they are more likely to go first and likely to get more attacks in.
While the Thief is not new to Final Fantasy, FFIII is the first game to allow them to steal items from enemies. When stealing, the game rolls a chance to succeed or not. However, Thieves can only steal one item and not every enemy has an item.
The remake overhauls the steal mechanics heavily. Enemies now have a table consisting of (usually) four items of different rarities. When the games rolls a chance to steal, it first looks at the Thief's job level to determine what tables to take into consideration. Then, it rolls on each table, with a failure resulting in it trying the table of lower rarity. This is especially notable as the Thief's table drops are heavily spread out:
Table 1: 1-30
Table 2: 31-70
Table 3: 71-98
Table 4: 99
As for the drops themselves, they vary in terms of usefulness. However, the later tables include at least one unique weapon (Odin's Gungnir, which otherwise has a 0.3% drop rate) and Phoenix Downs which can be stolen from the enemies on Dragon's Peak; This is the only source of replenishable Phoenix Downs in the 3D version of FFIII.
Thus, while it is ultimately up to the player as whether or not to Master the Thief job, there is an incentive to do so beyond the job mastery reward, unlike most other jobs.
The Thief job is also a unique job in that it has a passive ability: Thieves can unlock doors when at the front of the party. This is useful in cases such as the Temple of Time, which has numerous locked doors, as the alternative is buying magic keys from Gyshal.
When attempting to run from battle, the party is usually at the risk of double damage from enemies. However, the Thief has the "Flee", which does not carry that risk. On the other hand, the way the escape is calculated is different from the general "run away" command.
Pairing with Vikings and Knights is a smart move as Thieves don't have much for defense. Elemental attackers, such as the Black Mage line or Rangers are also useful. A Scholar may also be a serious investment due to the Thief's ability to replenish their stock of attack items. Offense-oriented thieves would do well to stick to the front line while Steal-focused thieves can hide in the back due to their ability to use throwing items.